For others watching this topic or researching this problem:
A few weeks ago I got my SM2 A350, but other obligations prevented me from setting it up right away. Then roughly 2 weeks ago I finally got it all assembled, and ran into this same problem.
I contacted Snapmaker Support via the website and started a ticket, explaining in detail each step I had taken in the troubleshooting and why that led me to diagnose it as a defective touchscreen (“breathing” lights functioning normally, reversing touchscreen connector had no effect, able to connect, communicate, and control via USB, etc.).
Snapmaker representative Zero Zhao got back to me in under 24 hours, confirmed my diagnosis, and shipped out a new touchscreen the next day. It took a couple days to reach me (I am in California, USA) and corrected the problem immediately.
Thank you, Snapmaker and Zero, for being so on top of this! I am extremely happy with my decision to buy a Snapmaker, the long wait to get the machine was totally worth it, I will certainly be buying the expansion modules as they become available, and I would absolutely reccomend this product to anyone.