l39,
As far as I am aware, there can be no installation of TCAS II that can work with just Mode A/C. TCAS II requires a Mode S transponder to co-ordinate RAs. The Mode S transponder is used to transmit Mode S messages on 1090MHz as the TCAS unit itself can only transmit on 1030MHz for interrogating. It is the TCAS II mandates in Europe that resulted in the early adoption of Mode S transponders prior to the European ELS mandates.
Can't help with the second question, I wasn't aware that you could seperate the TA/RA ON function.
RS.