I can't answer (a) as I don't know IST - sounds iffy though to me. As far as (b) goes, no recourse at all since it's two separate PNRs. However, if you find you have to cancel and rebook the connection, maybe this would be covered by your travel insurance policy (e.g. if you have a Plat/Cent AmEx or if you booked on your BA AmEx) - I'd check in to that avenue.

Phil