If you have the option to cure the mould with the plug, then this is always preferable. This means however, that the plug and the release agent have to resist to these temperatures and that you have to make sure you don't run into problems with different coefficients with thermal expansion.

If you post cure the mould free standing without the plug you need to ensure a very uniform temperature in the oven and very slow ramp rates. I typically use 0.3°C/min.

No, most automotive paints will be quite happy going as high as 150/160° C. The issue with them is even high solids 2k paints have a lot of solvents in them, and as the temperature goes up those solvents start to move around and the paint softens again slightly. Sometimes this causes issues, sometimes it doesn't. The bigger issue is if the panel being moulded has any body filler in it, then this will suffer and shrink badly above 60°, which will ruin the mould.

Personally, I wouldn't take a mould higher than 65° with the plug still in it, its a bit risky and fairly unnecessary. You are more likely to have big issues with CTE differences and pre-release and plugs suffering from the heat, etc. I will generally do an initial post cure at 55/60° for 8 hours with the plug still in the mould, and then demould and do a free-standing cure up to the final temperature with a very slow ramp rate (I do 0.1° per minute if I have the time, 0.5° if I'm comfortable or need to push it). The initial 60° post cure will give most resins a Tg of around 80, and then you can push the Tg higher without really coming too close to it in the process.