You asked several complex questions that would require complex answers;
I'll just touch some selected issues ..
First of all, majority of security cameras are CMOS with digital processing and then the signal is converted to analogue (if camera has component or composite video output(s)) ..
There are basically 4 types of transmission: [1]composite video over co-ax, [2]composite video over twisted pair, [3]IP network data streaming (These are web-like cameras with IP addresses, which send a continuous stream of pictures in the Motion JPEG format) over network cable and [4]wireless ..
Analogue transmission is not noise-resistant and therefore needs shielding, grounding, earthing etc. etc. techniques to prevent picture degradation ..
You have also asked about superimposing video signal and DC (or AC) ..
In theory it’s possible, but it’s impractical, so the video signal is FM-modulated and shifted from 0-6MHz to 6-12MHz and transmitted over coax together with DC, or wirelessly in, say 2.4GHz band ..
The RS-232 is not designed to operate over long distances and consequently the RS422 or the RS485 is used ..
See:
http://www.marcspages.co.uk/tech/long232.htm
IanP