Yeh, I think Methods isn't worth accelerating. Sure, you might be able to do well in year 11, but I'm fairly sure you can only do better in year 12 alongside Spesh without taking too much extra time. If you're going for the whole time-saving argument, methods doesn't take much time in year 12 at all as it complements spesh. If your brother is as good as you say he is, he probably won't need to study for the methods SACs or do much of the homework because as others have said, it will all be covered in spesh minus probability and a couple of other things that can very quickly be covered in revision. I personally didn't do many methods exercises at all and didn't really need to spend time revising for SACs because yeh, spesh covers all of it really. I didn't even do any Methods practise exams (keep in mind though that I did a hell of a lot of spesh ones so it's not like I walked into the exam without preparation). As for the 'MHS style SACs', there's nothing wrong with them. Particularly not in Methods at least (the spesh ones can be a bit bitchy at times, but they're always fair). Also, being weighted against the MHS methods cohort rather than a distance ed one for SACs is a nice backup.
But as for the being forced to do an extra subject thing, I don't think you are. Those who did external Chinese at MHS were allowed to only do 4 subjects in year 12 and got a crap load of free periods. It's really up to him whether he'll use them productively, and it's quite hard given that peer pressure often means you can't just go sit at the individual study desks, and end up at a table with all your friends and inevitably get nothing done.