What you find logical may be debatable as you would probably get as many answers as users there. One thing is sure, and this is what I just emphasized by my stupid joke to @DavidPackouz about collector Aeros, is that there is absolutely no reason for that button to be hardcoded to any specific action (Play/Stop All)… And I personally clearly agree to your approach which for me seems logical too.
Still about that looper mode, another thing that probably needs to be clarified is the role of the bottom left button.
You mention in the diagram that you can freely create parts and tracks there.
I think the behavior and boundaries should be a bit more defined, although I can see the power of the mechanism.
Another point about that button behavior, is that I guess this is only valid for an empty song, and becomes more a navigational thing in the scope of a stopped song, or do you imagine modifying “live” the TxP signature of an already recorded song (probably too complex) ?