Yes you can but you need to do some steps first. You should trim your hair with a hair trimmer like this one: link trim your hair till its really all cut, leave very little on your head then you can use the razor to shave it bald. I recommend you trim it extremely short and keep it that way, its very close to being bald really, rather than shaving your head off completely as razors can cause cuts in your head if you're not extremely careful.
Yeah, you can do it that way. That's how my ex used to do it. There were a couple times he'd run out and be due for a shave, so I'd bring him a razor from my house so he could get the job done, haha. Just make sure you buzz your hair really short before you attempt that though, and maybe even have a friend help with the back of your head if you have trouble. Use a good shaving cream and moisturize afterward with some kind of unscented lotion, that'll cut down on potential razor burn.
Yes you may use the same razor, however cut/trim all the hair first. I get the first shave done at the barber shop and then shave myself every three to four days. Shaving the head once a year is good for healthy and strong hair and there is never any dandruff problem.