How to re-route drain pipe before installing a new vanity cab?

I just rented a beautiful basement apartment in a fairly new home. When I looked at the apartment for the first time a noticed a faint odor but assumed it was because the place had been closed up for a while. I've been here two days and notice that there's a sewage type smell coming from the bathroom. At first I suspected that the water traps had gone dry so I ran water into each drain for about 5 minutes. Nothing changed. Now I notice that the smell is stronger when the people upstairs use the bathroom. Any ideas why this is happening? Thanks!

Installed a new compression fitting to sink, had to resort to the proverbial overtightening (read below why and what I may have done wrong). The new fitting was leaking (dripping ~3 drops per minute), then a couple of hours later it stopped leaking. Has been dry overnight. Is this normal? What should I do? Leave alone or try some remedy. I don't think I should tighten it any more. It's already overtightened. Details: The semi-flexible hot water connection to my sink started leaking last night (rubber seals completely aged and disintegrated-likely the originals from when house was built in 1990). This is how it was: [URL=http://s490.photobucket.com/user/ThomasE_1/media/Sink%20supply%20line.jpg.html][IMG]http://i490.photobucket.com/albums/rr270/ThomasE_1/Sink%20supply%20line.jpg[/IMG][/URL] Now, because the existing compression fitting (to 1/2" copper pipe) was the old multiple turn style (I have found that after a few years these never fully close and also have a tendency to leak from the stem), I decided to get a new 1/4 turn compression fitting. Unfortunately, upon removing the old fitting, I discovered that it must have been overtightened during the original installation (1990 when house was built). So the ferrule was quite stuck As a side note, I should have suspected this it because when I redid the compression valves in the kitchen sink I also found the fittings overtigtened and had a hard time getting the new ones to seal again. So, I tried to re-use existing ferrule and nut with the new fitting... but they did not seal even under moderate overtigthening. So (plan B) I just used an automotive puller to pull out the ferrule. Towards the end, I experienced a lot of resistance on the puller so I left the ferrule very close to the end of the copper pipe and partially cut the ferrule with a hacksaw, then snpapped it open with a screwdriver, being VERY CAREFUL not to dent the copper pipe. Indeed no damage to copper pipe. However, as you can see from the photo, the original overtightening seems to have deformed the copper pipe (you can see the saddle in the copper pipe diameter in the photo, and I imagine this is not normal -- it’s a sign of overtightening right?)) [URL=http://s490.photobucket.com/user/ThomasE_1/media/Copper%20pipe.jpg.html][IMG]http://i490.photobucket.com/albums/rr270/ThomasE_1/Copper%20pipe.jpg[/IMG][/URL] In any case, I put in the new ferrule and gave it a try, while already planning in the background how I may have to sweat a copper pipe extension etc. Sure enough after normal tightening (hand tight + 1/2 turn) the compression fitting was leaking quite a bit (one drop per second). So I tightened another 3/4 turn and the flow was reduced to about 1 drop a minute. I tightened a bit more thinking that perhaps it may seal, however after tightening another 1/4 turn the leak actually increased (Uh Oh, better look where that propane torch is, I thought,...) Well, I gave it another 1/4 turn and the flow reduced again to about one drop a minute. Frustrated, I left it as is, since it was 10pm and we had to cook and I did not want to end up having to shut off the water to the house. But then,... when I checked about 2 hrs later, the leak seemed to have stopped. This morning, things were still dry, no drops in the bucket either?!? Long story short, should I leave it as is? Or is this high risk for leaking in the future? I wigled a bit the fitting just to see if perhaps I had a very borderline seal, but it did not leak. I cannot explain this and wonder what to do, so I'm turning to the experts (or other DYIers who ended up in a similar predicament). Any advice appreciated. P.S. I'd also like to replace other (the cold water) compression fitting since I'm doing this, but I'd rather avoid the same complications and uncertainty.