More observations from working with captions in Premiere.
If you import a .srt file, then change your import settings, then delete the .srt file and re-import it, Premiere will apply the settings that were in effect when you originally imported the .srt. It doesn't seem to matter how many times you delete and import, Premiere stores the original settings someplace, ignores your current settings and applies the original settings instead.
I found that I had to change the .srt filename - EG adding a "2" to the filename - before Premiere would finally "forget" the original import settings and apply my current import settings. This behavior is the reason I thought changing my import settings had no effect at all on importing .srt files - because it doesn't if you try re-importing a file that Premiere has already imported.
I also just experienced the "no-changes-take-effect" issue - wherein changes I make in the "Captions" window - EG font size, background opacity, etc. - aren't reflected on the timeline until after I quit Premiere and restart.
So if you're pulling your hair out with frustration, please know that you it isn't you, it is in fact the application. Maybe some of these tips and workarounds will help alleviate your frustration.