(Long answers to short questions.)
Decoding an MTS signal is relatively cheap and easy. The greater complexity of the system was designed into the encoding task. The rational being that there are LOTS of TVs that will decode an MTS signal and relatively few broadcasters that will need to encode the signal. SO, make it easy to decode and don't care about it being hard to encode. Thus, MTS decode chips are plentiful and cheap and MTS encoding chips don't exist. Stereo VCRs only decode the MTS signal. They do not encode MTS.
The only sat receivers that ever had integral MTS circuits were a couple of C-band receivers from Chaparral that have been off of the market for years. ALL current VCRs, satellite receivers etc. are monaural on their modulated (ch3/4) outputs.
The MTS signal takes the left and right audio signals (L & R) and sums them together to create an L+R signal. Added to that, riding on a high frequency carrier wave, is an L-R signal (with a lot of DBX compression). So, now you have one 'signal' that has both channels of information. That is how a single RCA cable can carry both channels of information.
I don't know if this is at all clear. MTS works very much like FM stereo. (With a lot more noise reduction.) There are probably better descriptions of this technique elsewhere.