yes you can restore from backup. it will come on with everything you had on there at the time of backup.
to receive ota, wipe system, restore backup, and flash stock recovery. also lock bootloader via scotty123's method.
some people reporting ota working fine without locking bootloader. but stock recovery must be there for it to work
unlock without htc dev only possible with soff. so no on stock phone.
not gonna explain soff and unlock, read forums. explained there
to install rom, just unlock bootloader.install recovery of your choice thats it. the rom you install will probably be rooted anyway!