you dont need to drop the engine mount to remove the cam cover (you will need to to replace the timing belt however but from what I gather you just want to tighten the tensioner).
The top cover has 5 bolts in it - not the 4 that you have probably removed.
You will need to remove the crank pulley and possibly a belt tensioner or two to get to the remaining covers and tensioner pulley.
Personally I wouldnt bother putting the covers back on after that little operation.
While you have the covers off check the condition of the timing belt and change it if you need to.
A decent trolley jack with some padding on the part that makes contact shouldnt damage your sump.