Just because it's signed doesn't mean they have to support it since it isn't official firmware. Call tech support and tell them you're running the leaked MR2. I'm betting they'll likely tell you to revert back to MR1.
That's not possible (without rooting) and if it were possible to revert they would have everyone running MR1 back on the launch software, since MR1 is the cause of the reboots and they have stopped pushing it to new phones.
To the OP:
If you are running the launch software, don't update to MR1 or 2
If you are running MR1 and not having issues then stick with it
HOWEVER:
If you are running MR1 and the reboot issue is making your phone unusable or almost so then update to MR2. It wont hurt anything and you have everything to gain from it if you are having issues.
That being said I would not volunteer the info that you are running the MR2 update to tech support. You MAY get an uneducated tech who actually believes this voids the warranty.
If by some chance you have to send the phone back here is what they (VERIZON) check for:
1. Is the box it comes in physically damaged? Is the phone itself?
2. Does the phone have water damage (they check the sticker)
3. When they plug it in does it power on (does not have to actually boot, just needs to light up)
They do NOT check the software version. They do NOT check weather the device has been rooted or otherwise altered. Refurbished devices are all re-flashed anyway.