Good job with the definitions!
For the interfacing circuit I have been thinking of the cheap EBAY OBD adapters that use Bluetooth for communications to PC or smartphone. But because of the custom Honda protocol in the K-line it would require custom firmware to the adapter. There is some research done for that, for example (github pfalcon/blutunode) but building the FW is not so straightforward... The nice thing with this approach would be very low cost and ready made HW for interfacing the K-line.
I have made my own BLE adapter from 'Ruuvitag' and I also investigated how to use the cheapest possible BLE adapters. But these need custom interfacing to the K-line (which is not complicated though, but still extra work...).
Reference to my github project if somebody is interested (github atoid/DashBle).
However for the ECU flashing it might be best to stick with hard wiring, no wireless link involved