1. It could work if you have perfect matching to the telephone lines in both ends, so nothing of the transmitted signal is reflected back to it's own receiver.
I reality, the receiver will receive too much from it's own transmitter for a reliable function.
2. The other end will receive garbage.
3. You can do 1200/75 full duplex between two TCM3105's.
4. There is no symmetrical full-duplex mode, but you can reduce the data rate in any mode, so 150/150 is possible in the 1200/150 modes.
If you have a 4-wire connection between the units, you can have 1200/1200 full duplex since the TCM3105 receiver will not see it's own transmit signal.