1. If you don't get your first preference because you didn't meet the academic requirements, then yor second preference will be looked at. And if you've met all the pre-requisites and achieved over the Clearly-In, then you'll get that offer. If not, if you have a third preference, then that will be looked at etc. For more info,
here would be a good read.
2. The essential part of enrolment is doing the online enrolment to accept your offer. So provided you have someone to receive your enrolment package which will arrive a couple of days (at the latest) after Round 1 offers are released, then you should be able to accept your offer anywhere in the world provided you have an Internet connection.
Some courses will have "compulsory" info sessions and the like, but they're not really "compulsory"... In fact, nothing at uni is compulsory... And that's absolutely nothing.