1) The dive shop there says "save a dive" stuff so I’m guessing no rentals but call 1st. 575-472-3370 You need to download and fill out the permit before you arrive, pay there.
2) I’m sure PADI will sell you an Altitude cert, mine included it. If you are diving computer it will figure altitude into your times. The link for more info on "Dive Sites" above takes you to a page that has tables for the Blue Hole.
3) & 5) At "Dive Sites" above it says 44 members from DB have been there, maybe PM them and see if any would want to buddy with you. Otherwise ask a lot of questions at the dive shop there before the dive, I’m sure they have heard them all from us flatlanders before!