Just two comments
Re low side switching and shorting, if you use "unswitched" RCA connectors on the controller, you can always wire the 12V to the centre, and this makes it much more difficult to accidentally short out
I have attached a simple circuit i use based on an LM324 opamp
( as well as a finished unit to show its not complicated )
Three sections of the OPAmp make up a proper sawtooth output waveform, and the 4th section is used to provide a buffered power supply to drive the Mosfet.
By adding one more LM324, you can use the sawtooth waveform to feed 4 more supplies and hence get 5 independent channels out.
I would have to say tho, that after playing with multichannel devices like this, the cabling becomes a nightmare
What i am looking at doing is to make a very small PCB using SMT components, and actually build a controller into each dewstrap.
That way, all that is reqd is a simple 12V supply bus.
Andrew