The 45 degree decking is the primary sub-floor. It's a technique that was widely used in the past, but has subsequently been replaced with simply dropping sheets of plywood down.
In answer to your questions:
1) You can glue and screw to the sub-floor.
2) In this case, it doesn't matter. The sub-floor is carrying the load diagonally. However, if there were no sub-floor, then you would want to go perpendicular. See this discussion on the subject.
3) Always - always - always level as you go. Fix the level at the soonest possible opportunity or it can be amplified by an order of magnitude.
Stuffing the open area with fiberglass will absolutely help, as it will dampen/absorb the sound waves resonating between the joists. It will only go so far, though, as a significant amount of energy from foot fall traffic will still be transmitted through the floor and into the joists, causing the sound vibrations in the first place.
To combat that, you'll want to dampen the energy (convert it to heat) before it gets to the joists.
Step one is to add mass, if you are able to. If you can raise the floor a bit, then add one or two more layers of some kind of dense sheathing. Concrete board works great, but is expensive and hard to work with. 3/4" (19mm) OSB would work great.
Step two is to add a damping layer between the layers, to break up the resonant frequencies between the like layers. At minimum, something like 30lb roofing felt would work. A custom-created product like Green Glue would work even better.
Step three might be to add a final damping layer designed for foot fall traffic. There are quite a few brand names out there and they all work roughly the same, by maintaining a densely flexible layer just before the subfloor.
This site has some really good info on this topic. They are trying to sell you a product, but even if you don't go that route, the rest of the info is very solid: http://www.soundproofingcompany.com/soundproofing-solutions/soundproofing-floors/
Best Answer
Yes its an additional vapor barrier and could prevent underlayment from warping. In a bathroom its always a good idea to do to keep moisture away from subfloor.