Any competent TLDI technician will start with the Factory service manual as a guide, and then go into diagnostic mode and see what conditions are recorded by the ECU. Similar to what an automotive tech would do using the OBD2 connector of a modern car. Based on that information, the tech then takes electrical and mechanical tests/readings to fully diagnose the problem. Typically, the tech then replaces or repairs the issue, and the problem is corrected.
Long story short, there aren't really any shortcuts to doing a competent repair on a TLDI.
If you have average mechanical skills, decent tools (including an analog -- not digital ohm meter), and follow the troubleshooting section of the Factory manual, you should be able to work on the motor with confidence.
Dealers can get parts promptly -- we get ours from the Dallas warehouse by 2nd day air in most cases -- there isn't any reason to wait months, unless the part is not in stock in Japan, and has to be manufactured. Any dealer can provide you with the parts you order.