Well, a good appetite is important...a loss of it is usually a bad sign. There seem to be others on this board with so much more medical knowledge than I, but as I understand it, piroxicam is an anti inflammatory. So that in the sense that it would relieve inflammation, it would reduce pain. Tramadol is a straight pain reliever.
We would never treat a human with cancer with just piroxicam--they are not even sure how or why it works (except some theorists think the source of many problems is inflammation) and of course it doesn't always work as well as we would like.
As for the straining, you must do what you think is best, but for Annie, this is what I did: I take her to work with me (at home we have a doggie door--that's another story) and that's the only time she is walked. She would pee, it would turn to a drip and then nothing. We'd go back in and a bit later she's want to go out again and we would repeat that process--and so on. I found that if she stopped, walked a bit and tried again, she'd have success. So that's what we started doing--once she's in the drip mode, I distract her (actually I don't have to anymore, she caught on to it) and we'd walk a few steps and she'd squat again. We might do this three times before she'd have her bladder all empty and than she's be good for another few hours.
Don't know if this will help or not. Good luck to you
Marty