I had mine professionally done. It was a mess down there (dirt nearly to the floor joists!) and very expensive to have it done right. I can't offer much in the way of what should actually be done, but mine is now all lined with heavy duty plastic (no foam that I can see). I'm in the south though so I'm not sure if they did any insulating at all, but definitely none on the floor.
I would think you'd need to run it over the concrete as well otherwise how would the transition from concrete to vapor barrier work? Mine has a dehumidifier running down there as well. I recommend sticking a bluetooth hygrometer down there to track humidity from inside the house. Mine has been great for identifying when the dehumid goes out or something is going wrong. Good luck!
Ditto here back in November. Crawlspace walls first covered in 20 mil plastic (held on with adhesive). Then 2" of foam board insulation on top of that - secured with masonry anchors. Same 20 mil plastic to cover the bare dirt ground, with overlapping and taped seams. All piers have the plastic running up them skirt style at least 24". All gaps, cracks and penetrations (HVAC, plumbing, electrical) sealed with foam. Finally, since my HVAC is in the crawl space - a 4" duct was added to the plenum so conditioned air is blown into the space. To avoid a dehumidifier (expensive and one more damn thing to maintain) I had them install a drain to daylight drain in lowest spot of the space.
I too have been monitoring before and after with a remote for temp/humidity and it runs no more than 10% higher humidity than the house. But, this was done when humidity was running 40% or lower, its (quick eyeball) currently 69F/31% in the house (kitchen) and 64F/37% in the crawlspace. I'm curious what it looks like in Spring & Summer.
Looking at OPs questions ...
1. The plywood on vents may be overkill with your plans. Why not use the same foam board
2. The cinderblock walls probably dont need complete covering, just enough to keep moisture from seeping in where they meet the ground. Ie: running the plastic up 12-24" etc
3. Concrete is porous - moisture can get through. However, how much would need to be quantified - maybe do everything except concrete and measure over time ?