When a prov is being sat, both the owner and assigned sitter can log it and do things. The paid service should be equivalent to teamviewer, if not better. At present it's inferior to a free product which creates incentive to cheat beyond just being cheap.
Part-time players would also be less detrimental to kingdoms. I don't have access to admin data but I imagine the people most in need of sitting are the ones with non-office jobs who have significant blackouts in their availabilities. This class of players has a lower amount discretionary income to spend on credits while being functionally forced to pay double rates if they want to partake in parts of their wars while they're home (canceling sitting early and re-setting).