I doubt the rental Pokémon were EV trained, since, well, they were programmed into the game. EV training existed from the beginning, most of us were just too young to understand it. But even then, it wasn't really invoked since it was given to you fully at level 100.
EVs existed in Gen I and II, but the system wasn't properly developed until Gen III. One could continuously battle Pokémon and eventually max out all of their Pokémon's stats. See here for more details.
Theres no way of telling. and i dont think people complaining about how bad crits and stuff were proves anything..But I mean, ive beaten pretty much all the game with rentals, so they cant be too unfairly balanced.