That's not even remotely close to being true. I have no more trouble imagining what monolingual me would have been like than I have imagining what color-blind me would have been like, or deaf me, or paraplegic me.
Imagining a deficit in one's abilities is not hard. It's going the other way -- imagining what it would be like to be able to do something that you have never done -- that is difficult.
(In fact, I don't even have to imagine. I know what it is like to learn a language as an adult, and how different that is from learning one as a child, because I've done both.)
Of course. That is all anyone can do with respect to any counterfactual. So what? You were making a claim about the fidelity of my imagination:
"You have no clue what monolingual you would have been like."
And that's not true. I have several pretty good clues. Not only that, but what I imagine about the counterfactual me leads me to be able to formulate a testable hypothesis with respect to non-counterfactual situations, namely, that acquiring multiple languages in one's youth does afford some measurable cognitive advantages in general.
You can get a lot of leverage out of imagining things.
What benchmark could you possibly have for this? You have no clue what monolingual you would have been like.