Yes, yes, and yes. "Ever" and "even" both make the sentence more emphatic. If you forget it *even* happened, you forget everything about it, even the fact it happened. If you forget it *ever* happened, you mentally remove it from the history of the world, so it's not even a distant memory. "Even" focuses on what you do WHILE you are forgetting it--it focuses on the WAY you forget it. "Ever" focuses on the end result: it focuses on the way you look at the world after you have forgotten it.

Yeah, you can use even and ever there. There is often quite a bit of difference between the words, but in this circumstance, there isn't much difference. Ever is putting the emphasis on the tense aspect, having happened ( 发生过的意思) Even is more like 'actually', it's more emphasis on the verb happening. But putting emphasis on the tense, and the fact it happened, doesn't make much of a difference. A case where it would be more different would be below: Have you ever been to France? (where you are talking to a friend and you are asking each other what places you have been to,) Have you even been to France? (perhaps a person is talking with authority about France, but they are saying lots of incorrect things, you might respond with this) Even , definitely has the a connotation of dismissal. So in 'let's forget it even happened' you're kind of dismissing the importance of it, or implying that you should both dismiss it as not important. Again the 'have you even been to France' you're dismissing what the person is saying, you doubt they have been to France.