Upvote:5
a) Yes, you will have to clear Immigration, collect your checked baggage, then clear Customs, then recheck you luggage, then pass trough TSA to re-enter the terminal. 2.5 hours should be plenty.
b) No, you will clear US Immigration and Customs at DFW. DFW-SAT is a domestic flight.
c) You should get all three Boarding Passes when you check in with BA in Mumbai. If not, the Agent will tell you what to do. There is a BA transfer desk at LHR that can assist.