Promilus

@1 - yup, note some throwing weapons have slightly higher number of attacks (throwing spear just like bow adds +1 attack per round)
@2 - yes, BG1 version of character will transfer, so imoen thief 8th level/mage will remain that way. And Jaheira with 18wis also
@4 - nothing - only fighter-like classes get additional attack per round on 7th, 13th and then even further (with mods like tdd). Monk has special bonus. clerics, druids, mages, bards etc. are limited to 1 attack per round, and neither 7th or 13th level increases it, nor weapon specialization (2* or more).
@5 - if you dual him at 10th you'll have just 1 base attack + 1/2 from 7th level + weapon mastery (1/2...3/2 IIRC). After dualing to mage and getting fighter abilities back you can still grand master weapon you did specialize as fighter, but IIRC none other (I might be wrong at this). You will not get any other ability from fighter class... so no fighter HLAs after 3mln EXP. Just mage's.