question on the roof structure of my garage
In order: 

a) You can do it! You didn't mention anything about trying to move the existing structure, so this is based on my understanding that you are simply adding the tie back in to prevent further spreading. Personally I would use something more like a 2x8 or 2x10; a 20 ft. span is a long way for a 2x4, and you'll likely see some sag. I would put 5 nails into each rafter, in a rough square pattern with one in the center. But, you'll want to offset the nails a bit from each other so that they aren't going through the same grain (which can make it easier for a crack to split the wood where the nails are). You can cut the ends at the same angle as the rafters to get a longer surface to nail through. 

b) If you do end up hiring someone else, a handyman could certainly handle it. 

c) From your pictures, one tie every other is fine: the plates of the walls aren't going to move with a tie every 4 feet, and additional ones aren't going to do much.