Final bump. This is something where having an order in which group preferences are applied (e.g. asset map order) would be very useful.
From what I am grasping of the code, the preferences code in general prefers groups that are deeper in the tree. It uses getParents() to get the list of groups, and the default order appears to be that of decreasing tree depth (doesn't appear to specify what happens to multiple-links).
For instance, if you have a structure of two parent groups /A/U and /B/C/D/U where U is your user and the others are user groups, it will apply preferences at C before that at A, despite the latter being closer to U.
However--
There is one last wrinkle to this, and that is: each asset type's preferences are allowed to determine how their preferences are merged. For the Default Content Type preference, it will always prefer content types other than WYSIWYG when merging preferences from two applicable groups. So in the case you're offering, there is currently no way to override a preference for a non-WYSIWYG content type with a WYSIWYG Content Type selection, even if it would otherwise have priority.
I'm not sure why it was written that way, except for thinking that WYSIWYG [being at one point the only "free" content type, back in the Matrix 3.x days] would always want to be overridden with something, and didn't make sense for it TO override something else (maybe because they were considered equal priority). This assumption may no longer be valid (and I'm not sure it's even documented to work that way) so it may become part of our Squizmap to-do list if there's no reason why it should be that way. No guarantees though - only that I'll raise it.
Thanks for your question(s) Douglas; sorry if the current answer disappoints you.