You have your first couple answered so regarding number 3:
The character talking is the one who the other characters react to. So if you want to build relationships, your character has to be the one interacting. On a good note, this is actually beneficial if you know that an action you want to take will piss off a companion that you like.
E.g. If you know Shadowheart will be pissed about admitting you have a worm in your head, have Gale do the talking for that conversation instead of you.
Honestly, I wouldn't worry too much about 'winning' every skill check in conversations. None of the convos (in EA at least) lead to a game over, and letting your character lose some can make the game more interesting.
If you are really worried about it, there is an amulet with 'Guidance' you can grab early on which gives you an extra 1d4 to your rolls, and by level three Clerics learn a spell that lets you roll two dice during skill checks (known as advantage in DnD). Between those two, you get about a +6 on average to your rolls.
