Yes, water definitely plays a role in plate tectonics! I actually discussed the intramantle ocean in a shorter story of mine recently. A reduction in water could have definitely changed Venus' geologic activity. This is especially relevant with the recent finding that more surface water is subducted than previously thought, and how that may tie into Earth's greater hydrologic cycle.

I am very interested in Jupiter's role in the formation of our solar system given its mass compared to the other planets. We already know Jupiter's gravitational influence on our solar system has affected other processes and planetary development in our solar system, so it wouldn't surprise me if it changed Venus' development in such a way. I'll have to look into it more, sounds like interesting research!