For what it sounds like you're about to do, copying the entire XYZ project folder out of your My Paratext Projects folder into another location is probably sufficient. Then, if something goes wrong, you can simply delete the project and then copy that folder back into its original place. That's by far the easiest way of "backing up", particularly in preparation for a one-time change that you're unsure of.
Rather that "Mark Point in Project History", I would recommend simply doing a Send/Receive. That process also marks a point, but has the advantage of being a slightly more robust backup--if you do something catastrophic to overwrite the history data on your local machine, that backup would still be in the cloud. This process is better than just copying the folder out like it suggested in my first paragraph, but it's also slightly more complicated to restore things.
It is reassuring to know that it is extremely difficult to make a change that is so bad that someone can't restore an old copy of your project. I'd say it's almost impossible for you to corrupt data such an extreme, but not impossible for someone with access to the backup registry.
2 & 3
If you use the copy-folder method, you don't really need any privileges at all.
If you are trying to restore an old point in history, you may need administrator privileges (I can't remember off the top of my head). And if you made such a huge mistake that you can't fix it, you might need to ask for help from someone with access to the server.
4
That kind of depends on what change you made. But depending on what you're doing, you'll want to test this carefully before doing future editing on the project. It's extremely easy to delete a mistake and go back to a saved state a few hours ago. It's extremely difficult to realize after a month of work that some data is broken, but you've also made lots of correct changes in the meantime.