Re: Why simulate the TCAS?
TCAS unit doesn't know who care about where it is or how it's moving. It's just measuring the time taken to receive a reply, and the rate of change of that time to estimate closure rate, and the phase difference of the received signal on two antenna to get a bearing*. The reply it's listening for is literally the serial number of the transponder unit in the other aircraft.
There is nothing in the signals being transmitted or received that gives position information (altitude information is encoded in the reply). This allows non-TCAS equipped aircraft to be detected and avoided by TCAS equipped aircraft as pretty much all aircraft have a transponder fitted so they appear with useful information on ATC radar.
Consequently the only way to do spoofing would appear to be altering the time the response signal is transmitted. I suppose you might be able to transmit two signals slightly out of phase to confuse the bearing calculation as well?
*It's not that accurate in bearing but it's not really an issue.