Not sure about the rough idle but probably not a bad idea to do a MAF sensor clean and reset anyway
I believe that these cars suffer with vibration due tothe dual mass flywheel (mine has got a lightweight solid one so cant describe exactly the nature of this vibration as never experienced it).
If your temperature needle on the dash isnt going up to over half way then I would say that you are fine there tbh. My engine gives off a lot of heat too and I think its fairly normal. Does you rad fan kick in if you get it very hot and sit idling? Mine can take a lot to get it up to temp (well into the 90's°C) but then the fan will come on.
What makes you think the inlet manifold is on the way out?