Read that it always uses the highest skill possible even in dialogue? Is that true even with persuasion checks, so basically if you skip that skill on "charname" it will just use the persuasion of one of your npc followers?

this is exactly how it works. if your follower has higher skill - game will use that. I tihnk there are a few occasions where your MC has to talk with no followers at hand, but those are really rare.

That would invalidate classes that don't focus charisma and cha classes/builds are already ones of the strongest in the game - they don't need further boosts. DC's in the game are very high. anyone who does not have corresponding stat or feats will fail miserably more often than not.