Thanks! I'll try bending the derailleur, haven't done that yet.

As for the tension, I've losened it to the point of visible slackness in the cable and it still doesn't work like it used to. It becomes sluggish to up shift and remains resistant to downshifting.

While I would like to replace the cables and housing and clean the components, I know I did something wrong in my adjustments as well. I can be reasonably sure because I know the problem developed immediately after I worked on the biked, rather than developing over time. Even if the cables are shot and the derailleur is dirty, I would expect it to work at least as well as it did before with all the same parts and dirt.