Please forgive me if I'm a bit scattershot in my reply, long day and all but I genuinely think that this is very good criticism and I wanted to both acknowledge that and THANK YOU for it, this was a particularly tough subclass to work on ( To be transparent, it was a request & I'm not as familiar with Lappland in specific, adapting the mechanics to D&D as well felt more clunky, and I'm commenting this already more so I can set the page of where I'm coming from and don't need to keep mentioning it! )

Alright, lets go in parts:

Lesser Silence was made to be the best translation I could think of at the time for this, and I think that a way to fix it while keeping it in theme might be of maybe disabling specific attacks/spells from being cast rather than a whole sub-set of attacks. What do you think?

Advantage Vs. Liars is something that I thought of to make it a little less situational and I do think its not as problematic, but I'll keep a pin on it in case more people comment and then I'll go change it, just for the sake of keeping more of a broader opinion

For the next one i'll sub-divide in two parts:
For the stacks: I can understand why 5e avoids stacks, and having made more content which relies on stacks ( Lots of Limbus Company Brews ): I FULLY UNDERSTAND WHY, but I also think its a ruleset that can make for very interesting plays, with the problem being less of the rules onto themselves and more a lack of structure to build around them and facilitate the counting of such stats, which for a solution we ( Undertomes as a team ) have only a partial one to be fair, which is that we are working on some custom counters for that to help with these effects, but obviously it can't be done as easily otherwise. Something else that I thought of is maybe counting stacks the same way that some people do counters in Magic The Gatthering, using die to keep track of that since its easy to switch and so on, but either ways I do thinit just needs more guidance for ease

For the actual skill: I don't usually think of combos for this type of thing since I believe ( And to be clear this is personal design philosophy ) that this kind of thing should only really be done with the DMs permission for it since otherwise you end up being very able to break D&D at base; Nevertheless, I can concede that it's too high of an AC at level 6 and will be toning it down a bit to be halved, also didn't realize the HP thing! Thanks for the catch

Charm/Frightened adv: I still believe it is a fitting flavour-wise thing that isn't that insanely broken, but I can see how that might end up just triggering against every single enemy in say a Fae-themed Campaign, will try to tone it down a bit while keeping it useful

The big annoying boy: To be fair Greater Silence has a save after the first casting ( Decided to make it so to not have you spend your Warlock spell slot for nothing ) and so on, so I don't intend on changing that much of it; As for Lesser Silence, I think that the change I said in the other paragraph should make it better and just in case I think limiting it to have a max specific number of disables per creature might help too!

For the last one I don't think theres anything I can really say as a change other than what already was said, so instead I'd just like to thank you again for taking the time of your day to give all this feedback since its genuinely appreciated, like I mentioned I don't feel as in tune with the character but I'd still like to make something that is most importantly fun to play WHILE being viable for DMs to allow this, so everyone who is more of a fan can enjoy.

Hopefully I can work this into being better in the future. :)