As far as your temp gauge goes, you can drive a car with a lot of coolant missing, as long as your going fast enough the incoming rushing air cools the engine.
Happened to me on my E34, driving all day no problem, stop at red lights, stop signs, no sign of over-heating. If you stayed stopped for more than 1-2 minutes the temp would start to creep up and then at the 3/4 mark start to rise fast so I would start driving again and in a couple seconds the temp gauge goes back to the middle.
As far as the reason it could be any of those (except water instead of anti-frezze)....to check if there is a clog, take out the thermostat and put a garden hose through the opening and see if water comes out the other end at a steady pressure....if it doesn't it's a clog somewhere between the thermo housing and the bottom rad hose.
The thing about doing the HG on a V6 is that both could be finished and you wouldn't know unless you opened both heads. So it means doing 2 HG's.