The fact that everyone seems to be sitting back doing nothing, and letting a bunch of low level characters sort it out strikes me as a contrived scenario. Whether that's a module issue, DM issue, or both, I am not privy to.
That's pretty much the case with every FR adventure ever, isn't it?
Precisely. I picture Elminster sitting back drinking tea, scrying all the low level adventuring parties he has sent out to face certain doom and save the Realms from disaster. Like a spider in his web, he watches from afar and laughs at their piteous struggles.

Not to mention the fact that every other barkeep is a retired 12th level fighter, and you can't swing a dead cat without hitting one of the Harpers. But no, let's leave the saving of the world to this group of noobs.
Which is why, when our group decided we'd give this adventure path a try, we decided to make our own, very early version of Forgotten Realms for use in our rotating DM game. No one in our group is a great scholar of the Forgotten Realms, so we did not pay particular attention to the real history of the Realms. We just set the clock about a thousand or so years earlier to create a dark and dangerous world with islands of civilization (points of light). Using the Book of the Righteous, we created a great Church that is largely responsible for carving out these safe areas and founding the major cities. The trade routes are now expanding, and the merchant class is exerting increasing influence in the world.
The other major change we made to this world is that most people cannot rise above character level 2 in ability. Those who can are called supernals. They have been influenced by the gods, given a small measure of their power, although the supernals are not controlled by them. Supernals are the only people who can be magically resurrected. They are rather like all the Baal-spawn in the Baldur's Gate computer games. They have the potential to develop great power, and to do correspondingly great things. As Olivander says of Voldemort in the Harry Potter books, it could be terrible things, but still great. Many of them become adventurers, and are drawn to other supernals. Most of them wind up in songs and stories, one way or another.
With Photoshop, I amended and modified the map of the Sword Coast. We took a hatchet to all the factions present in the modern Realms, leaving various church factions, and influential merchant families. Neverwinter has been taken over by a supernal wizard who is the most powerful mage in the known world. He is building a school of magic, and fomenting resistance against the iron control of the church. They are very concerned and want to control him, but have no real way to do so. I have replaced the Red Wizards of Thay with his school.
The Cult of the Dragon has gone from a bunch of end-of-the-world crazies into an organized and dangerous force, and no one is really sure why. It has been difficult until recently to get people to take them seriously, but word of their depredations has spread, and the church has taken notice. Our characters took a caravan directly from the city of Whitehart (replacing Scornubel on the map), up to Waterdeep. I expect we'll make it to the city during tomorrow's session. In Waterdeep, we will finally learn that all of us are supernal. We already know where the caravans are taking the treasure because our group captured and charmed Freya (was Frulam) Mondath. I change names of people in these modules fairly often.
In any case, setting the clock back to a dark age where no one can safely travel long distances without risk, and the entire population is much smaller has made it easier to justify the lack of official action against the cultists until later in the story. I am running the main plot, my husband has been running the caravan portion, and my son runs the cities and the church.
Changes I plan to make include removing the vampire from the castle entirely. There's no reason for it, and the encounter is probably a bit too challenging. There are plenty of other things I can add. I am planning on introducing something like the draconian from the Dragonlance books. Made from the eggs of good dragons (many draconians made from each egg), they will add some tougher challengers for the higher level characters. Most humanoid races can't directly contend with the PC's, except for cult leaders, who are themselves supernal. One of the characters has the gold dragon bond, and would therefore be absolutely incensed to find out what they are doing with the eggs of good dragons. Although I suppose it could be *any* dragons. I haven't decided yet.
Clearly removing all the factions that are used in the module has changed it substantially, but we have built our own power groups to replace them. Most of the FR factions are too organized and too Renaissance or later in feel to be used in an early steel-age world. We have also kept magic fairly scarce, which I like the feel of. The power the characters have is intrinsic, and not dependent upon their gear. At nearly 5th level, the party has one magic weapon, a set of elven chain, a magic robe, a bag of holding, and two minor items. It keeps magic rare and special, which works well in 5E.