I was not aware that secret missions were tradable, especially on Facebook.
They aren't. Read the words immediately preceding what you quoted which said "specially chosen adventures". I.E. when I played adventures to level him up, I chose specific ones so that he would get secret missions. The facebook part was referring to the items.
maintain the adventure's spirit.
So adventure's spirit, not the spirit of the rules, but the adventure.
What counts as a house-rule versus an AL rule is, ultimately, up to the DM.
I mean, not really. The FAQ is very clear about which variant rules are allowed. Any thing beyond rules as written is a houserule, and while I know that AL tables use these house rules, such as 5-10 diagonal movement on a grid, flanking, critical miss, 20's on skill rolls auto-succeeding, etc... they are not AL rules. They aren't game breaking and don't transfer, so it is largely ignored. But any DM who uses them is breaking the AL rules.
Just because you have a '100% legal' AL character doesn't give you the right to make the game all about you or your character.
Who said that's what is going on? Wanting to have a min-maxed character has nothing to do with wanting the game to be about me. In fact, if I show up to a table where I'm the only optimized character, I find it very annoying because I don't WANT to stand out, but I also don't want to play a sub-optimized character just because other people aren't as good at building effective characters. I want a challenging game, where everyone is as strong as they can be within the rules and uses good strategy and teamwork when taking down challenging encounters. With my fighter, I'm happy to step back and let the social character take the lead in social situations. But I'm not going to gimp my fighter to keep him as weak as other players, especially at an Epic.
Your Curse of Strahd DM was right to be upset, because you and your friend were being dicks -- though what you were doing was not illegal, it was problematic for exactly the reasons you state, and if you ask any admin, they will say that such play, while allowed, is discouraged -- especially if (as seems likely) your DM specifically asked that you not play other adventures while participating in the Curse of Strahd game.
He did not specifically ask. He was annoyed, he realized that it was the trade-off for playing within AL, and he moved on. And in the end, he was actually grateful because otherwise the whole table would have been under leveled since the hardcovers do not provide enough combat XP to keep you at the recommended levels.
if, however, you insist on being a dick at my table, you shouldn't be surprised if you find your 'fun' to be less privileged than that of the other non-dick players at the table.
If having a min-maxed character is being a dick, then so be it. I am very happy to let others come up with the plan of attack, to engage or avoid combat as desired, lead the social situations. My fighter is an apathetic veteran mercenary. He isn't blood-thirsty or seeking combat. But he is a good fighter. I wanted to make him the best fighter he could be within the rules, and that included seeking out items that complement his abilities. Especially since I pretty much only use him in Epics and to play online in-between epics to boost him as needed.
It's not against the rules, but nothing in the rules says the DM has to let any particular player play at their table. It's a social contract thing, not an AL rules thing.
Then don't run it as an AL game at a store. If you aren't interested in the portability that makes AL what it is, then you can play the hardcover as a non-AL easily enough. In fact, even easier since it gives you more control on giving out XP/milestones.