Well I had to try the BLR-BOM-FRA-ZRH option today as all flights out of BLR were full. I am afraid that it is not a good option:
a) BLR-BOM on AI 621 at 20:20
Check In was ok, went to domestic ( BLR-BOm ! ) security and was sent back after clearing security to International. At International they refused to let me in as this is a domestic flight. Finally he let me in " hope you know how to make your way to domestic". Attention this flight is not on the displays!
Although BP showed Gate 19, it left ( ontime ) from Gate 12. Keep close contact with the lounge wardens ( no they are not dragons ). Arrival on time at BOM at 21:50. Flight was 14/14 in C and only 7 Pax in Y.

b) Transfer at BOM Terminal 3 to Terminal 3
They don't know that you are coming in domestic either. Ask, ask, ask and you will find the right channel. Bags were on the belt when I arrived. The walk from bag claim in this glorious airport ( you stop complaining about ORDeal) to LH check in is quite an experience as you are arriving with all int'l widebodies and leave with all int'l widebodies. It IS crowded!

c) Check In
Check In is ok, then you will spend an hour or more in lines for passport control and security check. Make sure that you hand luggage is correctly tagged or they will send you back. Security is strict as in TLV!

d) Lounge
You will spend then approx 4 hours in the F or C lounge ( C is less crowded ).
Nothing to write home about this lounge, it is plain boring! No work space and gets crowded starting with the 00:50 departure to MUC and then BKK, JNB ....
Standing room only around midnight. Your FRA flight will not be boarding before 02:30!