I think you're misunderstanding me. The antithesis to the idea of "Not talking to The Ur-Mother for any extended period was the best possible move in that situation" isn't "The Ur-Mother was a total pushover for the Forebear that he crushed like a ant." The narration is pretty clear that he slept because he spent mightily of himself. But here's the thing. The Forebear had the equivalent of Scion's PTV, he could cheat by stealing the power of ruin, and he could Cut Through her attempt to force him to the negotiating table via imposing stasis on casualty. We know that because he quite clearly did that in the flashback.
What I'm not seeing is any sort of textual evidence that had he just talked with her for five minutes, perhaps learned about the thing that had turned his life into a ceaseless procession through over a thousand hell worlds, that he would have somehow been robbed of the ability to do so.
People keep saying that had he not immediately went for the kill her super social powers would have totally mindwhammied him, but and I can't stress this enough, she had the chance to, they were actually talking with each other, and she couldn't manage too. Even the barest attempt to get him not too immediately rev up his successful attempt to kill her after he didn't immediately capitulate in full surrender did nothing to stay his hand. Hell, he didn't even bother to turn off his hearing during the second encounter. He literally kept offered her a last chance, and her response to that wasn't to try to social him to wait and hear her out, it was to oppose him in direct strife which funnily enough caused her to kill countless innocents.