1. Yes it's okay, and with that film and a 72 I typically do EV+ 8 stops or so. In a desert I think I'd go for EV+6 or so, depending. I'd do a test sheet (see #4 below).
2. f/32 is going to get you into some really long exposures with that film, and pretty soon you're off into reciprocity issue. If that's okay then fine, but it isn't necessary. Just review your hyperfocal technique and recall that the distance to which you focus plays a very big role in DOF. I.e., above all, know the hyperfocal distance for your setup. Now, with IR, you want to focus a bit (few percent) closer than what your eyes instruct you to do. But with this particular film and the 72 filter, you're actually not getting that much light beyond ~700nm, not nearly as much as you'd get with HIE or such. So the refocus will be relatively minor, and yes it can usually be taken care of by stopping down one stop or so. By the way, I have used various inexpensive apo process lenses and find no need for refocus from the UV (403 filter) through the IR (#87 filter). Especially if the subject is a good distance away. Anyway you're shooting quite wide, I don't think you'll need to stop down to f/32 unless you're doing closeups. Maybe for your Welwitschia you will indeed want to stop down a good deal. Just don't focus too far in; good hyperfocal technique goes a long way with these near-IR films.
3. Keep all film in a dark bag as much as possible. When you withdraw the darkslide: don't pull it out all the way, or if you do, make sure the opening is facing downward. Use a dark cloth to cover the camera and darkslide as much as possible.
4. desert shooting or at altitude: the air will probably be much drier than you're used to, so don't be surprised if you exposure compensations for the filter are much less than you might need at home. Here's what you can do: take a test frame each day and develop that first when you get home. You can use that to optimize your development for the other frames. Don't be shocked if you need a 2 stop adjustment in development!
P.S. Oh and don't forget to put the filter on after you compose I've got a whole lot black frames after a scene got me excited...
P.P.S. Welwitschia looks like an awesome subject for IR! Enjoy!