I did year 10 entry (didn't get in.)
The principal says that if you get top 5 there's a high chance you'd get an offer, if you're in top 10 there's a smaller chance and if you're outside of top 10, then it's unlikely. However even if you get top 5 you still may not get an offer as it depends on whether current students move school, interstate etc.
No one knows how exams will run this year unfortunately, the situation is rapidly changing so it's too early to tell.
All the schools you apply to will use the same test, so that means you'll sit it once at one of the SEHS that you have applied for. I don't believe there was a preference system in Year 10 though, and I'm pretty sure that even if there was a preference system, your chances of getting into MHS wouldn't change.