"... as to date it seems nothing that is Teensy has needed hardware or other debugger support ..."
Ehhhh ????
It seems to me that the only thing really missing is support for a jtag / swd programmer / debugger
If a single operator has created it, I think it would be very simple for PJRC designers to create an interface for the classic Seeger debugger or similar
And that's the only way to get the flow of the program under control
I purchased a teensy 3.5 and a 4.0 with the intention of experimenting with the possibilities
And given the potential of the cards, it seems really incredible that it would be impossible to program and debug at the level of the single line of code
The Arduino ide does not allow to proceed quickly nor to check what happens during the program flow, it is not an environment to use if the requests are beyond the classic 'hello world'