I would definitely check on the taxes, but I would recommend either Lufthansa or Emirates as opposed to doing the CO option. The international terminal is not connected to the domestic terminal in BOM and you will have to take a bus there. (You'll have to clear immigration and customs, and recheck your bags, unless something has changed recently.)

You'll likely not have lounge access at the domestic terminal. (unless the Kingfisher lounge will accept that you came in on a BF ticket. I have no experience with that.) Even then, those are very long layovers to spend in BOM, and unless you know your way around Mumbai or have family there, getting out of the airport to see the sights isn't really a option. (plus it'll be late at night anyway.)