I'm 42 and went through this kind of **** with my dad.
Dad's philosophy--and he told us this often--was "If you can survive me, you'll be able to survive whatever the rest of the world throws at you."
It's a ****** way to grow up. "You can't do that/you'll fail"-prove-him-wrong *****.
"You can do anything you set your mind to if you work for it and want it bad enough" is a hell of a lot better way to do things.
Sounds like your dad might have the same ideas on parenting as mine. Never outright throwing me under the bus, but damn sure didn't mind throwing obstacles in my way. He's 64, and we can't work together on anything.
If I were you, this is exactly what I would do:
Fit that "little" shed of yours out exactly like you want it. Sounds like Grandad might be of some help, too.
Insulate it if you can... there are cheap ways to do that.
6x13-- you can light it with a pair of 100-watt bulbs. just keep them on in the winter, and your insulated shed will be remarkably warm from the heat of the lighbulbs. Keep them on year-round, and it'll cut down on the humidity in there too.
Point out to your father that with your bench and tools out of the garage, the lawnmower will fit in that area, with room to spare.
Put a padlock on the door to the shed... if he asks about it, tell him you're obviously worried about someone breaking in and stealing your tools.
Maybe there's some work you can do for your grandfather to earn a little money for insulation and white pegboard. Wallpaper that shed with white pegboard. You'll love it.
Don't forget to maximize the space in the roof--storage, shelves whatever.
Go online and look for small woodworking shops... it's amazing what some guys fit in very, very small spaces.
After you get a job and earn some money, you might be able to build yourself a one-car garage, or a one-car addition on the machine shed.
for now, move out to the little shed.
heck, if it were me, I think I'd set up a radio, a little fridge and sling a hammock across one corner!
-Brad