Also:

http://www.dannychoo.com/post/en/585/Gundam.html

http://modelling-abc.blogspot.com/2011/04/top-coats.html

Explains that my panel-line smudging may be fixable by spraying very thin layer first, waiting a day, then finishing.
Yellowing may have to do with the temperature (slow drying is better), but may also have to do with the pigment in the spray? Test spray out on white pieces from box. Also some people talk about not topcoating at all, saying all topcoats yellow (eventually). Consider switching to modeling-specific.