Slaad Host (continued)
The module proposes that the slaad burth forst during the test of isolation. "Hey, Jim, nice you could make to the game... let's start where we left last week, you were in the test of isolation... suddenly a slaad explodes from Hurlaxes' chest. Roll initiative. BTW Jim, no replacement character will be able to be introduced while the character finish exploring Aurile's lair, which could be a memorable event in the adventure. That's... sad.
Owlbear Whisperer
Tough one. So you have a knack with Owlbears. I don't know why it would even be a secret. Do "dog persons" in real life hide that fact? OK, it's not exactly a wild animal, but I can't fathom why one wouldn't want to mention it. It's a kewl power! I can see a "fun side" where the players is tempted to confess the group (composed of a cannibalist pirate, a doppleganger and a secret cultist of aurile) "I have a dark secret to tell you... I have a knack with taming owlbears".
Pirate Cannibal
This is the only reference to subsistence cannibalism in the module. And it's sad. If your group has is strongly biased against anthropophagy themes, iit could lead to intra-party conflict and should be avoided in the game. On the other hand, if your group is OK with that, so much more can be done with it, but in that case it wouldn't make the pirate secret a strong secret. I'd remove the cannibalism aspect. Pirates were hanged, historically, that's a good reason for one not to arrive in the Ten-Town saying "Hello, I am a pirate and if your food shipments are late, it's because me and my crew often plunder them!" Making it a basic "I'm a pirate" secret is good enough. You will be hunted if discovered. Better give a "bonus side" (information on the Dark Duchess) and a penalty side on top of the difficulty to explain the source of this knowledge to the rest of the party. Maybe rewrite the story to say that you're the legitimate owner of the Dark Duchess and its legitimately plundered riches (that could even be a STARTING QUEST, just emphasizing that the player would need a strong party to recover the treasure...) but you really don't know where it is exactly, only the general areas. Complications could arise as... the crew traveled north and disbanded... taking a few prisoners with them for food in case things went awry... they drew lots on whom to eat each day, and
when they crossed a road and found a way to get to civilization, they still had a a survivor. They offered him a chance to live, calling him "Lucky" and sent him on the other way from the road. Lucky survived to get back the Ten Town as well and could recognize the character. It wouldn't need to be a real threat, but the prospect of meeting Lucky again should frighten the character. Pirates being democratic, they voted on whether to eat prisonners, and so the player isn't forced to accept a character eating people. He could even try to reach for Lucky and apologize, on the risk of being discovered himself. A cool complication could be at Revel's End if the character investigate there, they could be noticed by an inmate, a former pirate who was sent there until his trial because the Ten Towns had other, more pressing matters to deal with. He could threaten the characters to reveal who their friend is really, and want them to organize his escape in exchange for his silence. For an added moral difficulty, said pirate could be the one who voted AGAINST eating people and was in the minority, along with the PC if his player chooses that. Will the PC let Prisonner 299 risk revealing their dark secret? Will they risk standing against the most powerful force in the Dale to free him?
[Revel's End has many problem, ranging from the fact that it's obviously in regular (monthly at least?) contact with all sort of political powerhouse ranging from Mithril hall to Waterdeep to Silverymoon... and failed to report somehow about the sun not rising, to the fact it's the holding a garrison of 75 veterans, which is a combined firepower superior to the rest of the Ten-Town combined together... but I have ideas for grim-ify them and the tall tells, as well].
Reghed Heir
You were sacrificed to Aurile and survived. Mommy won't be pleased to learn that. Two ways to deal with that:
a) Aurile wanted a sacrifice and the goliath druidess defied her. The character should reasonably be hunted by Aurile's fury ever since. I'd make the character especially sensitive to cold damage, especially likely to be attacked by the beasts and so on... until he discovers the truth about the circumstances around his abandonmnet and try to Atone, as in the former edition spell. It would make interesting for the player to try to appease Aurile WHILE fighting her (like when the Greek were fighting Apollo around Troy, including physically, but atoned for them behaviour insulting Apollo's priest Chrises). A druid of Sylvanus could be the information provider on the origin of the curse and let the player have the choice... live with it or... maybe kill a "draft evarder" in Bryn Shander? Make it a very bad guy to help ease the consciousness of the player... but it's cold-blooded murder anyway.
b) Aurile accepted the sacrifice and sent Oyaminatok to save his protégé, that she adopted as a mystical child after being abandonned by his blood mother. The character is consecrated to Aurile ever since. I'd make the character enjoy the same benefit as a Midwinter child and make him able to CONVINCE the goddess to leave by making a plea for the inhabitants of the Dale at the end of the heroic journey to her lair. The same Sylvanus druid could recognize Auril's heir among the character's group. Only him could implore Auril to move norther to preserve the beauty of the world (by freezing it more)
elsewhere. She could however still ask for something, and she would certainly hate the idea of the Ythryn mythallar being reactivated as it could be used to stop winter altogether at some place. She could task her mystical son to destroy it, enticing the characters to continue the adventure after her encounter with Auril (which, knowing players would probably result in an hostile confrontation and the removal of the curse...)
Reincarnation
It should be linked more to the story to be meaningful. Let's look at the causes of death: avalanche, murdered by goblins, murdered by yetis, murdered by white dragon, murdered by a trout, sacrificed to Aurile in Targos, murdered by a doppelganger, murdered by a moose.
So you died, and reincarnated. You are free to chose your life prior to your death, and the subsequent reincarnation occurred to recently for your new form to have an impact. So basically, this secret is of no impact on your background: you (maybe) changed race at some point, but you could have been raised from the dead (an not uncommon occurrence in the Realms). Why would it be a secret? "I have a dark secret to tell you, my friends, and I hope you'll not see me in a bad light after this but... I had my tonsils out".
I wouldn't keep "murdered by a doppelganger" if there is a player with the doppelganger secret and I'd want to avoid intra-party conflict. It's too easy to devolve into a "you killed me, prepare to die". If the players are on board, it could be interesting to have one of them having an interest into keeping the presence of doppelgagner unknown and the danger to have an hostile doppelganger at large. It would be necessary to have a doppelganger subplot with a specific goal to be foiled. The Zhents could have sent him to help, including removing the Harper's spy who could be transmitting informations (and potentially target the potential Harper's spy among the group, to make it more personal).
Coming back from the dead after being sacrificed to Aurile would be fun, it could be treated like the Reghed Heir case. It would also introduce a nuance into the human sacrifice conundrum. If sacrificed people are taken into Aurile's care and
actually better off being dead, it's debatable whether it is moral to prevent sacrifices. And the character could have been sent back to be the herald of this piece of news. Of course, non-believers will abound. Whether the PC is right about the sacrifice is of course left to the DM...
Ring Hunter
Proposed use in the game: the character finds it in the ending banquet. As it's a secret he probably didn't mention it during the same, so 90% of the players will just end the campaign with a strange event where a party member chokes on a ring a decide to keep it as it was "a long lost family heirloom".

I'd remove it altogether.
Runaway Author
Another variation of the "you're tracked by a powerful enemy". To make it distinctive from the two other ones, I'd emphasize the fact that their hunter is supernatural. The outraged nobles of Baldur's Gate have asked the infernal powers to hunt the author... I'd keep the idea from the module that the knights of Caer-Dinaval could help, because I like the idea of offering a pact with devils. I'd make the devils increasingly harassing : (a) the character could be warned of his impending doom by way of the 3 devils appearing in a mist forming out of a book (or a shelf of books) to tell him to prepare for his death, will say he'll be punished for his work as a writer. (b) Next time the character is in front of a book or in a library, they'll reappear and quote that "literature is the reflection of life" (Lewis Carol) animate a copy of the character out of the mist and fight him (use the character stats). (c) Next time, they'll quote that"literature adds to reality, it does not simply describe it" (C.S. Lewis)... and make the reflection of the character formed by the mist and attack... with twice the HP and a fire-breath attack. (d) the devils will reapper, animate a copy of the character... and quote that "Literature is strewn with the wreckage of those who have minded beyond reason the opinion of others" (Virginia Woolf)... the copy flee, running to the town square to commit random murders of townspeople (especially fun if you established the doppelganger lore). At this point, the character will want to avoid seeing their reflection. Be sure to mention bookshelves in reflection, often and needlessly, in the hope that the poor writer will end up being a major book burner or avoid at all cost entering a room with books. At some point, the characters will be tempted to take the offer of the knights of Levistus who can then offer to lift the curse in exchange of a service. Or live miserably forever as writer, unable to approach books out of fear of being struck by the devils.
Having all character having meaningful secrets would make all of them grouping up together as heroes of the Dale unlikely... On the other hand, having an "interesting secret" and several "mundane secrets" may be less fun for those who didn't "get" the interesting one. But here are my ideas on how to darken the secrets. Do not hesitate to share ideas, on this theme or on how to darken the rest of the module!