Let's make the broad assumption that you've used the 'search' feature here and have answered all of the other questions not mentioned in your post. The other stuff has pretty well been asked and answered. You do have some unique questions. I will say that there will be NO problem getting any advanced instruction on Bonaire.
The cheapest diving is shore diving, and that might mean what you are searching for when you say "cost effective".
Here's my patented Bonaire shore diving tip:
Rig a two liter soda bottle, water filled with all but 1-2" of air, tie a 10' string around the capped neck and a borrowed 3# weight on the other end. Begin your shore dive, going out to the edge of the sloping reef wall, and as you decide which way the current is going (so you can begin your dive by heading into it), leave the weight and impromptu underwater buoy right there. (Make sure that you have just enough air inside to float it- not float it away)
I set the weight in 20fsw, thus the pop bottle now is floating about 10' below the surface.
When I return from my up current start and now down current return, I get myself in 20fsw bottom and besides enjoying my dive and safety stop/swim, I scan for the pop bottle. That's where I parked the car!
An extra benefit? Get a clear one and Sea Turtles and Baracudas might be attracted. (Turtles think it's a jelly and Baracudas like the flash)
My other shore dive tip is standard anti-bandit procedures. Roll down windows, nothing left in vehicle, station non-diver on hilltop with high powered scope, etc.
Have fun, eat goat, dive lots! It's in my top 3 islands of the Caribbean! :dazzler1: