The intent of the aspirational art time contracts I believe at for those starting to wind down their careers. Not that everyone on them is in that boat but I believe that was the original intent.
The right to request contract is exactly that. You have the right to request part time working. BA has the right to refuse the request. However I do not believe that they have, yet the lead in times are quite long at the moment. At peak I believe they were 18 months from request date.
75% is 3 weeks on 1 week off ad infinitum. The 1 week off starts on a Thursday.
The 50% is 2 weeks on 2 weeks off.
Leave, duty free weeks, pay, bonus etc are pro rated.
There is talk of an 85% contract but that is not yet in place. That will mean you will work to 85% of the monthly individual pilot workload (cap) but with no fixed days off. The fear at the moment is that if you're on 85% you'll work just as many days as those on full time but you'll get all the crappy low credit (hours) trips.