You would just go to the standard theme in that scenario until you can either obtain one for 4.7 or create one for 4.7. You could update from 4.3 to 4.7 if you wish, but there would indeed be quite a lot of changes
I think you answered your own question in the first line there. It really depends. For example, if you want to make a copy, its going to depend on whether you have a place to copy it to, the speed you are able to copy, and your own ability. To give an example of how impossible a question it is to answer, some may be able to copy their site in less than 5 minutes, where others it might take them an few hours or so to get a copy up and running.
That would test if a new install works, not if an upgrade of your platform works. One thing you need to bear in mind there is its not 'if' the server is set up in the correct PHP version. It most certainly isnt at present (4.3 would not be working if it was). Its checking to see whether it can support it.