1) Is there a level cap in the BG1 part ?
2) If you don't have NPCs in your party when going from BG1 to BG2 will they have their BG1 or their BG2 stats ?
3) If I make Imoen a thief10/mage will she still be a thief7/mage when I get her back in BG2 chapter 4 ?

Max Level for most seems to be 8(without cheating) Yes, as far as I've seen And yes, I do believe she'll still be the same

With BGT you get the Throne of Bhaal level cap (unless you also use a level cap remover) so even in BG1 you can go over the limit for that.

Not sure about number 2 as I've not got any that I didn't have (first playthrough)

As for number 3 - it definitely keeps the stats that you gave them (handy as I used a wisdom tome on Jaheira and dualled Imoen later too)