Does a ball try to load up the shooter lane and into the magnet pocket and then just dump out onto the playfield? Or does this not happen at all?

The motor screw going back and forth across the playfield is a normal part of the initialization process. Some video of what's happening would be useful.

OK, if the ball is loading into the pocket and then dumping out onto the playfield, it's usually because the ball-on-magnet opto is failing (I think this is switch 82?). I've seen this happen because of a board failure, but if you're still seeing this with the Homepin board, I would actually check to see if your optos have failed. (This has absolutely NOTHING to do with you changing the batteries). The problem is these optos are on opposite ends of the playfield and a total pain in the ass to get at (the right transmitter requires removing the right ramp and plastics....the left (receiver) requires removing the large castle plastic and then some finesse surgery.

That all said, the flasher interference error needs to be address. That COULD be the optos as above, but could also be that other playfield lighting is causing too much interference with the optos and tricking them into thinking a ball is there. Were there any recent lighting changes on this game? Your video seems to suggest some SUPER bright flashers.

I'd reach out to Ryan Wanger with COMET pinball in Boulder. He does solid tech work.