A great way of telling if the subframe mounts are REALLY shot (like mine are):
If you jack up the front end by the frame so that both front wheels are off the ground (car is 100% supported by the frame up front) and you look under the hood at the mounts (you will need to take the battery out to see the right side) and see any light coming in from around them, they're gone. You need new ones. My car handles very well but does undoubtedly "Float" over some corners/road conditions where going up a short incline and then going down one (the main one I can think of is the Scranton Expressway in my area, right after the Main Ave exit... PaulC would probably be the only one who knows this one...)

As for the fuel filter, the pump and filter are together on a bracket that's attatched to the body in front of the fuel tank (and in a little "cubby-hole"). You may have to take it off the mounts (I don't know, my mounts dryrotted apart long long ago) to access the filter, but it's pretty damn easy - take the lines off, loosen the clamp (machine screw), take the filter out. Installation is reverse, naturally. A 10 year old with a broken hand could do it (Especially if *hint* you drive the rear up onto ramps or jack it up (SECURELY!)).

The other seals, although I haven't done them, look pretty straightforward (I WANT to do them sometime soon - mainly the oil pan seal). I don't know how hard it would be to do the oil pan seal though without jacking the front up by the frame (and still I don't know about ease, I'm afraid of the subframe being in the way - you'd have to ask one of the guys here who have done it).

As for the idle issue, it sounds like an issue with your throttle position sensor not picking up the closed condition at idle, and the idle has been adjusted for this. If you fix the switch it may idle worse, but when you fix it it might help. Otherwise, I'm as clueless as you on it (but that's where I'd first look!)