I also just noticed that you mentioned the Subaru dealer said something about an ECU-to-immobilizer communication fault. I am no expert; however, it got me thinking that something may have happened to the original ECU prior to you owning the car (given its not-so-clear history). To my knowledge, there is a separate immobilizer unit ("control module" - part number 88035CA030 for 2014 BRZ Limited) that communicates with the ECM (part number 22765AG511 for 2014 BRZ Limited MT). Using the ECU removal guide from the thread linked by mav above, you can pull the ECU and check its part number and year (if present). I'd also look to see if there is any evidence that the ECM has been removed after factory (broken or bent clips, marks on bolt heads, etc.). It is possible that something nefarious happened to your 2016 ECM and it was replaced by a 2015 ECM (containing this unclear "2015 BRZ profile") and something has occurred to prevent it from pairing properly with your immobilizer, such as a prolonged battery disconnection at the tint shop.
This hypothesis of mine has some holes - like why would the 2015 ECM ever pair with the 2016 immobilizer, causing problems earlier - but its at least an idea. By checking the ECM location, you can also see if perhaps something has been damaged, either by water or otherwise.
I would check your OBD-II port (located under the steering column on the right side next to the knee panel). This port is extremely fragile and is often a source of poor connections for those of us who routinely use it to change ECM maps and track powertrain parameters (FWIW, the common recommendation is to get an OBD extension cable and permanently leave it attached to the car). Get an inspection mirror, light, and look at the pins, making sure they aren't bent. Check the bracket and make sure it isn't too bent out of shape, making a connector not seat properly. Without looking at the readouts of the dealer tech's code pulls, it's not possible to determine if they are even getting a good connection. One would presume they are, but in my years of experience, I've learned that the best way to ensure something is performed correctly is to do it yourself.
Finally, I would check the immobilizer unit connections in your car. The unit should be located in the steering column, even if you have push start (see this picture:
http://www.ebay.com/itm/2013-SCION-F...271258068708); however, don't quote me on this. While unlikely, it's possible you have a simple wiring issue for some reason or another. Always worth a check. To see how to get access to the steering column, search the forum for a thread about wiring the combination switches for fog lights, or for removing the steering wheel).
These recommendations should be free to do yourself (no special tools required besides a trim removal tool and a couple spanners or sockets). If I was in your position, I know I would feel better just having a quick look at these things. That said, if you are planning on pursuing legal counsel, I would heed the advice of others and remove these threads so that you have the best chance of getting a favorable resolution. Finally, if you choose to remove threads, please feel free to PM me if you want additional (but certainly not "professional") advice. I am no mechanic/technician/engineer, I am actually a physician, but I do really enjoy learning from and helping others, particularly about cars!
You can reference the parts using the diagrams shown on the website below. Information for newer model years is not complete - hence why I chose a 2014 BRZ Limited to pull part numbers.
https://www.subarupartsdepot.com/par...iagram=viewAll