Using Attic/Loft Space Above Garage for Storage - Adding a Subfloor
If the cords on the trusses are larger than 2x4 than it sounds like they used storage trusses designed to carry a little more load than normal.

1) No don't worry about another vapor barrier especially since it is a garage. The first one helps keep moisture out of the insulation which degrades its ability.

2) Good idea. A small gap allows the plywood to expand and contract with humidity and temperature without bowing.

3) If you are walking on the plywood then I like 5/8" since it doesn't spring as much under my weight (225 lbs), and even more so if you are storing items with significant weight to them.

4) Decking screws should be fine. I would go to 1 1/2" but you are just trying to keep the wood from shifting. 8d nails would work fine too.

5) Plywood and OSB have a higher strength to them lengthwise to span the rafters and carry the weight of the roof. So, yes lay the plywood or OSB perpendicular to the trusses. There is a good discussion of strength orientation here and they say plywood is generally stronger in its long dimension unless otherwise marked.

Good luck!