Learning Midrah with the Learned Lay Person

Note: Many smart people, after all, learn midrash in the way I am about to denounce, and I suppose there isn't anything wrong with it per se. It's just that this approach is a pet peeve of mine, and I had the misfortune of being trapped in conversation, recently, by one of its practitioners.

Learned lay person: It's a fascinating thing...

DB: Uh oh

LLP: We know Moshe had a deal with his father in law, that one of his sons would be allowed to practice idol worship...

DB: We know?

LLP: Yes, refer to the mechilta. So the question is, how could Moshe make such a deal....

DB: Actually, um, the question is why are you so certain that this happened.

LLP: I told you. Refer to the mechilta...

Something similar happened a few months ago, where me and the LLP were discussing the appearance of the three angels to Abraham, our forefather, at the start of Parshas Vayerah.

The verse tells us that Abraham instructed Sarah to "Hurry! Three se'ahs of meal, fine flour! Knead and make cakes" but, according to someone who's name I forget, Sarah had begun menstruating that morning and therefore was forbidden to touch dough. So how could Abrham have asked Sarah to prepare bread? (I should note that I am relying on my RW Yeshivish friend for these details. I don't recall ever being taught that a niddah must not touch dough, or that Sarah's menses returned before the angels arrived at Abrham's tent.)

When I tried to address my friend's question in the obvious way, my answer was waved off. The idea that the laws of Niddah were not known to Abraham could not be accepted by my friend even as a possibility and he clenched his teeth angrily when I accused him of thinking ahistorically. In return, he accused me of dismissing the question, by which he meant only that I was refusing to indulge in speculative weavings about what may have happened 4000 years years ago in Abraham's tent, when it seemed perfectly obvious to me that whatever it was that happened, Abraham certainly would not have reacted to it like a post-Talmudic Jew.

Suffice it to say, the whole conversation was a dead end.