#2 - Seems strange to me you would get better efficiency without a shroud. Conventional experience is the opposite. Lift eFoils typically come with shrouds and lots of owners remove them to get better performance.
#3 - I think two blade folding props like this are prone to more vibration and imbalance. The Flipsky prop is one of the cheaper options available, I think this is just a symptom of that.
#4 - I haven’t used this VESC yet, but I know a its commonly used in foil assist builds. I would just keep an eye on temperature. You could run a water line from the nosecone up to cool your aluminium plate on the top side above the vesc (no pump, just pressure from forward motion). As long as there is good contact between VESC and plate it should be fine.
underwater battery enclosures for an eFoil: This reminded me of the Lacroix Vortex efoil product. Similar ideas to what you want to achieve