1) No - (as mentioned first by Gardyloo below) if you start in IST you have to finish in Turkey. And remember that only two out of LHR, DXB, LHR & HEL can be stopovers.
2) I believe so (but I'm not certain).
3) Yes.
4) No, but the HEL-DXB is Y only crushed into a 757. The OneWorld timetable shows HEL-MIA as an MD-11 so I would have thought that would have business class.
5) There doesn't seem to be much experience here of doing it in IST, but plenty in other places. Levels of expertise in the ticket offices vary enormously. You may need to be very conversant with the starfiles. And patient.
6) Yes. It might be possible to do the spade work with a BA office nearer you and then get them to set it up on their system and hand it over to IST. But at the end of the day it has to be ticketed and paid for in IST.