Finally I got the complete Tribe Expansion Action loop complete, as you can see from the ugly gif below:

So now, after triggering the action, selecting the target region, and choosing the decision option to allow the tribe to expand, the tribe will actually start expanding into the selected region. Thought the process is slower than I expected.

In addition to this, I made a few tweaks to the action so that now it requires neighboring regions to actually border the tribe's territory in order to be selectable as a target for expansion. If no neighbor region mets this requirement then the action will not be available for execution.

While working on this I found a bug that sometimes crashes the game, so I'm currently trying to fix it. Beyond that, I want to change the UI a bit to improve the presentation of the Tribe Expansion Action, And I also need to make sure that the Tribe Expansion Action can be triggered somehow by the AI tribes, which will probably require another two to three weekends of work. With that I finally be able to call this particular feature complete, and move back to finish the tribal aggression feature which I left halfway done.