What You Want & What You Need
Wants are nice. Needs are non-negotiable.
Wants can follow preferences while needs follow your situation and circumstances.
If you want to have a particular external examiner maybe you can make a case to your supervisor. If they say no there will be someone else.
If you need your viva to be in an accessible room for you then that’s where it needs to be. No ifs or buts.
Put your focus where it needs to be as your viva is arranged. Try to get the things you want but if there’s something you need make sure that the right people know.
When you meet your needs you probably remove the worst of the stress that something as important as your viva could bring.
Make sure your needs are met then work to get your wants if you can.