Is it possible to calculate roughly how many games you have on a champ by looking at mastery points?

Roughly? Sure. Exactly? No. The amount of mastery points you get depends on your grade in a match (possibly as well as your teammates's grades) and if you won or lost, and obviously those are going to be different from game to game.

For a rough estimate, play like 10 games on the champ and record your mastery points in each one, then find the average. Divide your total mastery points by that and you'll have a rough idea of games played.

(That said, third party stuff like op.gg also can show how many games you played on whatever champ directly, so theres that too.)

Too many Variables. You wouldn't get a very accurate answer.

their would be a ton of variation in said players win rate. But I think mastery is also determined partially by game length.