So if Mozilla decides maintaining gecko isn't worth it, and switches to blink too, then it's basically just Safari and Chrome, and both would have to agree on standardization?

It’s kind of already the case.

Functionally mobile safari is the only thing stopping google having complete control of the web platform, just as MS did in the days of IE4-6.

There are a bunch of standards that google has proposed that are horrific for user privacy, and I would fully expect all google properties to become dependent on them if they didn’t have to care about mobile safari.

I think it's worth distinguishing between "shell" browsers that simply wrap an encapsulated rendering engine (such as those IE6/MSHTML-based browsers from decades ago) and other browsers like Chromium/Edge/Opera which have a different level of integration with the rendering engine - because those browsers are able to tweak and extend - or disable - the engine's features on a granular scale.

It's better to compare Chrome/Edge/Opera as "distributions and customizations" of Chromium/Blink instead of mere as wrappers - the same way that Linux distributions aren't reduced to just "GNU/Linux kernel wrappers".

No, from the point of view of the engine there is no meaningful difference between the engines.

Maybe they change some of the defaults, but none of those various wrappers are shipping engines that are different.