Hello,
Regarding your 'issues':
The learning curve for whatever you use will be fairly steep, so I'm not sure how important this will be?
If you'd prefer to use something on your Mac, are there not midi programs available for that allowing the editing of midi files. I'd guess so. They may not SAY anything about XG files, but they should still allow editing of such files, which are still midi. OK, there may be some things that might have to be done manually, and the software may not help as much as dedicated software might, but it will still be possible. Check this out, at least.
It's 'really old' - yes, I know the feeling. So am I, but I like to think far from 'past it'. Yet! XG is 'really old' too, but you're wanting to use it! I've got my trusty MU90r module here, and I've got some really nice XG midi files which I enjoy, so it's also a LONG way from 'past it'.
Geoff