I'm not sure I understand your question, but I'll try to answer it. Yes, you'll still be able to use Illustrator while a plugin is in development. It's possible that the plugin won't load at first, which doesn't stop you from using Illustrator for normal work. Once you're farther along it's possible that the plugin will load and may be a little unstable, but with Illustrator CC crashes rarely cause much lost work because its data recovery works so well (just make sure it's enabled in preferences if you're counting on that). I find that having actual work to do in Illustrator is a great way to test a plugin. If there are occasional issues, I run it in debug mode and when the next problem occurs it's much easier to see what went wrong and fix the bug. If it does happen to cause problems of any kind, you can always move or delete the plugin from Illustrator's Plug-ins folder.
I hope that helps.
Best of luck with your project!
I think they're talking about having the long-running process in a background thread. I'm not sure if that's possible, probably depends on what you're trying to do in the background thread.
Oh, that makes sense. If the question, then, is whether a user can continue to do other work in Illustrator while the plugin runs in testing mode, I'd say "yes" since in both Mac and Windows one can run a second instance of Illustrator.
Probably not, if your plugin takes into account the "structure" of the file you are working in.. layers/groups ... locking stuff changing transparency etc... those could change from user interaction and possibly confuse the script/give a different result.
i.e. if I lock the top most layer it will probably get mad it can no long access some text layer I was applying the plugin to.
I am not sure what you plug-in does exactly... but if it needs a document state to 'do stuff' that might be problematic while the user drifts around the file changing/adding content.