1 - Does Raydons media server software install on all Foxsat versions of firmware
AFAIK Using the current raydon's bundle 4.0.4 it will overwrite an old Humax Foxsat HDR firmware with the current Humax firmware 1.00.17.
If you are trying to use an older Raydon's Bundle version with an older Humax firmware I don't know what would happen.
3 - Once Raydon's software is installed on the Foxsat can it easily be removed if the box needed repairing under warranty
I'm pretty sure that it's easy to get the Humax back to a factory state. You have to delete
Take a look at this post and this one.
Take a look at the readme that comes with the bundle it has a lot of helpful info in it.