Are There Any Roles Where Getting An S Is Impossible?

Not roles, but certain champions are only played by insanely good people which means that is is super hard to get a good grade for them. Ivern, for example, has an extremely small set of mains who do super good on him and getting an S on him is almost impossible. As a jungler main with level 7 on nearly every jungle champ, I can't wrap my head around how to get an S on Ivern.

There's a few other examples. I collect level 7 champions so I'm not like a one trick god at anyone. Fiora, Riven, and a few other champs are pretty hard for similar reasons to Ivern but you can get lucky with them once every so often to get an S.

No

No, although typically playing a champion in their preferred role is harder to get an S in since the grading system compares your performance as that champion in that role with other players playing the champion in that role, meaning the one tricks for that champion that get absolutely insane scores are going to make it extremely hard to get a score that is within the treshold to be an S.

A tip to get an S as a champion is to find a role they can play and do effective in, but isn't played often or much at all. If you can carry the game or do really well, it's an almost guaranteed S as you are being compared to an extremely small pool of players and if you are doing even remotely good there's a high chance you will be within the threshold to get an S. Don't pick the champion in a role you know they can't do though as that's counter productive and will likely make you get a low score and make you lose the game. Ex. Playing Bard in a role other than Support that he can do decent in (like Top) can net you an easy S if you do well. Playing Ivern as a Support can net an easy S since he can do decent there with his shield spam. This especially works well on Red Side since he can scoop up Blue Buff whenever his allied Jungler kills it once he has reached the correct level.