#1
I'm Getting a PRS Tremonti SE Singlecut, and it has a Set-In neck, I'm worried about the neck breaking and me not being able to replace it. I know my acoustic is a Set-In Neck, and I know its replaceable, but I'm not sure with electrics.
#2
Usually with a neck break on a set neck, you just repair the break rather than replace. It's quite rare to have a break so bad it can't be fixed by a competent luthier.

I wouldn't worry about it. Get a decent case and sturdy stand, and take care of your instrument. It probably won't ever be an issue.
#3
The cost of a neck reset or replacement would likely be more than the guitar is worth. I'm strongly in favour of bolt-on necks in acoustics, as they are liable to structural deterioration, but I don't worry about it on electrics. They are stable, and you just have to avoid accidents, as noted.