Windows can be added to each copy of Grub; there may or may not be a way to add other copies/types of Linux, I don't know, but it doesn't happen automatically
Usually you can. The tricky part is that I was effectively installing two copies of the same OS in parallel, which an automated installer can't really do in the same place without name/reference conflicts
The consensus of Ublue / Red Hat / OS installer / general OS development people here regarding a lot of things I have done was "nobody ever does that, I have no idea"