Thanks for your interest! FYI there is a new thread about this topic [1] with the next version of the patch and more commentaries (I've created it for visibility purposes, but probably it also created some confusion, sorry for that).
Thanks!
I made a very quick look at your updates and noticed that it is intended to be simple and some parts of the code are removed as they have little test coverage. I'd propose vice versa to increase test coverage to enjoy more precise cost calculation and probably partial grouping.
Or maybe it's worth to benchmark both patches and then re-decide what we want more to have a more complicated or a simpler version.
Good to know that this feature is not stuck anymore and we have more than one proposal.