BOM also has a domestic terminal on the other end of the runway (just like DEL) and a shuttle bus provided just after the luggage belt so you can change terminals without leaving the airport perimeter. MAA is only airport I have used in india where the intl and domestic are in the same building.

FRA/VIE/ZRH to BOM are day flights, arriving in BOM around midnight local time, so seating is not a big issue as your internal body clock is still awake.

I would suggest flying directly to BLR. Most major EU airlines operate to all major indian cities, so going thru BOM and DEL is no longer necessary. I'm taking the new LH service to CCU on Dec 20. New C and everything!

Not flown BA longhaul for a long time, but colleagues who use it to fly to CCU and BOM are happy about the seating in C and complain about the food ex-India. VS I have no clue. Jet service is supposed to be quite nice, but seating old C from SAA.