Question 1) I do not know if this will help, and I just installed a replacement sink pop up assembly myslf. There is a nut (mine was a bronzish color) dirctly underneath the plastic spongy area that runs flush with the under sink. If you tighten that nut really strongly the leak should decrease. My sink is old and we don't have the funds to replace it now. My guess is that the actual sink area around the stopper area gets worn away with time so any new replacement part will not exactly fit. After tightening the nut I applied a water tight clear glue meant to seal boat leaks to fix this latter problem.
Question 2) I also have a plastic trap attached to the metal pop up I installed. I had to re-tighten it several times and also experienced leaks. My bet is that sizes change over years (like a size 2 is now a size 4
and pieces do not exactly match. I also put that same water proof glue for boat leaks around this area and it seems to work. (This can be found at any Ace-type hardware store). I hope this helps you smile and hold the line until you ( and I
can do a more major replacement.