Anycubic i3 Mega bed alignment / issues / tweaking

In which I find that the bed / y-axis of my Anycubic i3 Mega moves in mysterious ways, and try to fix it. 10-point thumbscrew caps on Thingiverse: https://www.thingiverse.com/thing:2497480 Follow-up video after 1 year of ownership (plus upgrades): (TBC)