Bosch EDC17's (like the LML Duramax uses) are not all the same, there is maybe 40 or 50 different EDC17's used with many different manufacturers, whilst the core design might be similar the software between them is certainly not interchangeable. Even the switch from E86A to E86B the ECM hardware and software is completely different.
Unfortunately TIS has been putting a lot of restrictions on its use lately, in fact the trouble you ran in to I'm not sure how a small time repair shop could get that ECM passcode as it is a dealer level thing as far as I am aware. With the restrictions and protection put in place by Bosch/GM on the E86 people have had to outlay $10K in Euro tuning hardware to get a base starting point. Some of the tuners out there with that equipment can provide you the exact base tune for a small fee.