First question about thaco

I play a human fighter/thief. In BG1 I found all the books with her and in Bg2 I made her a thief as soon as she got a lv9 fighter. Her base thaco as a lv9 fighter was 11. She got it back when she got a lv 10 thief. So far everything is perfect. But it stayed at 11 until she got lv19. All other chars (also anomen and nanila, also dual class) thaco improved in the meantime several times, even though I got more lv ups.
Does the thaco of the new class only change at a lv up when the thaco of the new class is better then the thaco of the old class?

Second question about exp:
In Bg2 my char (the fighter/thief from above) gets more exp than the other npc.
Often I park my group somewhere safe and kill the enemies alone with the backstab, run and hide tactic (exept undead groups, this is for minsc, boo and the mace of disruption alone). He (minsc) is #2 in exp.
Here is the question: How are the exp distributed in my group?
When I kill a monster:
-all group members get the same exp?
-everyone on the same map gets exp?
-everyone on the same screen gets exp?
- everyone who attacs or cast magic during a fight get exp?
-the closer you are to the dying enemy, the more exp you get?
-the more damage you do to the enemy, the more exp you get?
-the one who does the finishing blow gets more exp?
-some thing completely different?

Is it the same when you get exp from a quest?

Fighters and Thieves develop THAC0 differently. Fighters develop it faster than thieves, so what you are seeing is perfectly normal.

As for XP distribution, it is dependent on characters' prime requisite. I don't remember the excact rules, but for Fighters the PR is Strength, while for Thives it's Dexterity. If your character has at least 15 (I think) in all his prime requisites he gets a 5 or 10% XP bonus. Thieves also get personal XP for disarming trap, pick locks and pick pockets. Apart from that, generally all XP from combat and quests are evenly shared among the living member of the party. It doesn't matter who kills the monsters.

Each fighter level up reduces base thac0 by 1 so a level 9 fighter has a thac0 of 20-8-12.
Every two thief level up reduces base thac0 by 1 so a level 17-18 thief has a thac0 of 20-8=12.

For multiclasses and finished dualclasses the best thac0 of the classes is used, thieves have to reach level 19 to get a thac0 better than 12 (which level 9 fighters have).

Your party always get 10 percent more XP than displayed and it's shared evenly, leftover points are shared from top to down (only the character portrait order does matter).

So if your party of 6 defeats a monster worth 30 XP, your party gets 33 XP, 33:6=5 remainder 3, so the first 3 characters get 6 XP and the others 5 XP.

Or if you disarm a trap and get 2,750 XP displayed, the party gets 3,025, the first character gets 505 XP, the others get 504 XP each.

In the end the difference won't really matter, the XP differences within your party were already there when the characters joined the group or were caused by characters not getting XP because of being death, stoned or temporarily outside the party.

Always? Do you mean extra XP due to prime requisites does not matter?

Thanks for the answers.

My char has 19 str, dex and con (18at start and+1 from the books).

The leading char is always keldorn (because max cha)

Exactly. If you play with Minsc in the party you won't notice him getting less XP in spite of his monstrous wisdom.

I checked it in the game.
I killed 2 monsters (9000exp for each of them where shown on screen)
each of my 6 chars got 3300exp, thats 10% more than displayed.
I play on the standart D&D rules difficult setting

Well, that's a bit disappointing.
Bug or laziness?