Though it wasn't clear to me who proposed this deal to begin with (maybe I didn't read it closely enough??).

It's of a piece for Trump. He wants to do the fun part of being in charge (Have great deals! Make big announcements!)....but doesn't seem keen on putting in the work on the hard part of getting to those agreements (Understand the politics of your coalition! Recognize and mediate disagreements with the opposition!).

Since he hadn't done any of the work in preparing for a bargain on immigration, he shot from the hip. The people who were in front of his face seemed like they could reach a deal - so he told them to reach a deal, and he would take the heat if people in his coalition didn't like it. So they went out and, in two days, reached a deal and brought it back to him.

But in the meantime - surprise surprise! - Trump actually had some more conversations and learned that immigration was hard. There were people in his coalition that didn't like this deal! And they were mad that he might do the deal! He hadn't bothered to understand that his coalition included some people with very strong opinions on immigration, and that he would actually end up taking some heat on the deal. That's not the fun part of being in charge!

So the deal blew up. Not because of Dianne Feinstein, but because there's a group of Republicans who would never agree to any deal with Democrats that wasn't maximally restrictionist on immigration, and one of Trump's chief aides on immigration (Miller) is firmly in their camp.