Ok first off... the thing with the window... yes... it would be a good idea.. and it would fix the problem by replacing your weather strip. (the rubber part your window rides on)
The seat belt thing is just a reminder... not a light that stays on untill you have your seat belt on.
I would say buy a new fuel gauge... if that doesnt work.. I would go back to the sending unit... (underneath your rear seats) and check the connections there.. and research it a bit.. and see what is where.. and start replacing stuff... that is the only way to really do it. Sorry.
Unless someone before you has put a newer clutch and or a racing clutch in your car... it shouldnt really bite at the last possible release of the clutch by your foot. I would get that checked out.. it could be a number of things. Could be your clutch going.. could be the pedel... or the linkages between the pedel and the clutch. I'd have it looked at.
Exaust... its probably just a hanger... they come apart all the time... easy fix.