As an attorney you’re a trained negotiator. There will be a modified form of Remain in Mexico. Know why?

Because Lopez Obrador wants Joe Biden re-elected.

Not as much as AMLO wants to remain in power. The reason there won't be a significant re-establishment of Remain in Mexico is the same reason that you think (correctly) that this is a big election issue for Biden. Having lots of asylees hanging around the border for a few years, without the ability to legally work, is a huge burden on the government and those communities. Right now, that's a huge burden on the U.S. AMLO suffers politically if it becomes a huge burden on Mexico instead.

Biden just doesn't have much to offer AMLO, since big changes in U.S. immigration or economic policy require Congressional approval (which will not be forthcoming). In fact, AMLO might correctly calculate that he's got a better chance of a better deal under Trump, rather than Biden - because Trump will be far more motivated to be seen as the guy that "fixed" the border than Biden. Sure, as a defensive political maneuver Biden would like the border to not be as salient an issue going into November - but it's never been one of his political priorities. Meanwhile, there's probably no single policy issue that's higher on Trump's list of priorities than to be the man who fixes the border.

So if what AMLO really wants is State Department support for his efforts to bypass the Supreme Court, neutralize the Instituto Nacional Electoral, and cement Morena as the new PRI....well, Trump would trade that for border control in a heartbeat, while Biden never would.