Just a made an attempt to downgrade the firmware but failed for some reason.
0. Extracted the MP6,1 firmware from the install disk of macOS 10.13.6 (MP61_0124_B00.scap) from July 2018
1. Installed clean copy of macOS 10.13.6 (just in case)
2. Executed the following command in the Terminal:
Bash:
sudo /usr/sbin/bless -firmware ~/Desktop/MP61_0124_B00.scap --verbose --recovery
Output attached. Shutdown. Something was definetely happening for a minute or two, several reboots, but booted again with 429.60.3.0.0 (supposingly reverted to it because of the --recovery option).