>instead of providing one of the few ways to fight against the full monopoly on the web by Google is plainly absurd.
Firefox becoming a better browser and gaining market share is what will fight against a monopoly. Whether Firefox does that while using Gecko or some fork of Blink it doesn't really matter.
I fail to see how monopoly is avoided by using a Google driven rendering engine.
Because that rendering engine is open source. This means whoever is shipping the browser has control over it.