my 73 is the same as you describe...flare fittings, etc. not alot of room in there, but yeah, we'll both have to do some modifcations to get a by-pass kit installed. (its on my list, too
). It might be easier to use pex connections, but I'm not certain that there's enough room in there. check out www.pexconnection.com;
most new RV's are plumbed with this stuff...more resistant to freeze damage, and easy to install w/ compression fittings.
I wouldn't put the pink stuff into the water tank, as I hear it is difficult to get the stuff out completely. its not "toxic", but I'd just as soon do the bypass valve and then pump the antifreeze directly out of the gallon jugs. they also have a kit that permanently installs in the water pump's intake line, with a valve that re-directs to this auxilliary line that you put into the gallon jugs of AF.
If you open all those low-point drains like you mentioned, that'll drain the water heater, as well. one of them is for the hot water line.
I'm definatley doing the anti-freeze method this year, as I had damage to my pump last winter, when we had particuluarly cold weather. (sub-zero). I followed the manual's instructions, like you said, and then I blew out the lines with air, as well...still had damage. there's no way to get ALL the water out. and those few drops that remained in my pump were enough to break it.
The other devices you describe sound like the check-valves. they keep city water pressure from going to the pump, and pump water from just flowing out the city water connection. there is a built-in pressure regulator in the one near the city connection, but its still always a good idea to use the external one, as it protects your hose, and gives you a good backup for the built-in regulator. (who knows when that one will fail, or if it already has?).