A common way to play Imoen is to dual-class her into a Mage after about lvl 6. THis is how her character starts out in BG2: a dual-class thief/mage.
And lose your thieving abilities for the party for more than half of the game? This is not common, and only viable if you have another thief in the party.
Both Viconia and Branwen make excellent meat-shields as they can wear any type armor
With only 8 CON, Viconia makes a TERRIBLE meat shield.
One more thing: if you play through BG1 in its totality, you will find potions that boosts your character abilities, plus some magic weapons that boost them even further. This makes certain power-gaming character builds viable. For example, start with a dwarf as your PC having 19 Con. As you explore you will be able to raise it to 20, and your dwarf will regenerate HP automatically each round.
Regeneration from high CON does not restore HP every round, at least not to any meaningful degree. It takes so long that only during resting, or traveling will you notice any difference -- it is NOT like a ring of regeneration that restores HP every 6 seconds.
IIRC this character build can level-up to a 9/9 fighter/cleric under the vanilla exp cap (exp needed to level for both fighters and clerics is pretty low).
Nope. The highest possible fully dualed Fighter/Cleric is 6/7 -- a 9/9 fighter cleric would not only have the fighter class still inactive, but would need 475,000 XP, when the cap is 161,000.