With regard to commas, one of our working assumptions was that tag names might legitimately contain commas, that’s why the tag adder does not treat a comma as a commitment for tag creation. That said, we could make this an option, however, if I understand it correctly the only real difference would be that you’re hitting a different key, right? Instead of typing <tag name><Enter><tag name> you’d be typing <tag name><comma><tag name>, yes?
Grouping tags for quick access is an interesting idea! We’re currently refactoring the tag selector (the main motivation is improved handling of large numbers of tags, but this will involve some other changes as well) so that’s certainly something we’ll discuss, thanks!