Can it expand to 42"?

No. If your preferred strap is 33 to 37" in length, the Short is for you. Otherwise, choose the Long (38-45") or the Extra Long (42-54").

is the 33"-37" measurement for just the strap itself, or does is it the entire thing from end to end?

My son's strap is about 39.5 from one end of the leather to the other. The 33" measurement is from the inside peg hole on each end, with the strap adjusted as short as it will go. But if, you let out the strap as far at is will go and use the outside peg holes, I get that the strap will extend to about 40". He's a young teen and this is the only strap that is comfortable enough for him to use to play for an hour or more, standing.

Will this strap help with neck-dive?

My experience is: it depends. Why might it help? The strap is wide, so the material will grip your shirt. So, if your neck dive is minimal it helps a great deal. If it's severe, it may not. I had an Ibanez RD500 that had some neck dive, and this strap solved the problem. I now play a Spector, which is very well balanced. Neck dive is no issue, but I still won't give up my Comfort Strapp due to the comfort.