Thank you for posting the video and photos. It helps a ton...
It sounds like a loose piston in the bore to me.
When I look at the photos of the piston skirt on the exhaust side, there is a BUNCH of scuffing. That in turn, reduced the diameter of the piston at the skirt area, which in turn has increased the clearance in the bore between the piston and cylinder.
The rocking of the piston in your second video is very normal. The pistons are actually tapered slightly, to compensate for expansion when warmed up.
I see NO CHAMFERING at the edges of any of the port windows... anywhere. That's a total no no.
You mention about 20 hours on that top end. I would still see "cross hatching" from the honing in the cylinder walls if that was true. This to me looks like a piston was put in a used un-trued bore. I'm not saying that's what happened...just saying that is what it looks like.
With that amount of scuffing on the piston skirt, It looks to me like that cylinder was installed dirty and un-cleaned when assembled, or no air filter was used, or a damaged air filter with a hole in it was used, letting in excessive debris.
All of this is repairable. You need a ROUND, STRAIT bore with NO TAPER and .003 clearance for that forged piston, and PROPER CHAMFERING of ALL PORTS in that cylinder.
Not yelling, just saying.