Bet it's a bit redundant to hear about these lifters. I'm no mechanic and this is probably the biggest project I've got into so far on an engine. I've followed the steps you laid out in your video and I'm just not having any luck... It's it possible that it's too gunked up or am I lucky and it's something worse than my lifter. Can I put something in there like trans oil to help free it?.. anywho, gonna try and attach a picture, I'm worse at technology than being a mechanic lol.. I'm a board butcher, so any tips help.
Was the rocker actually floppy loose? If so then you do likely have a stuck lifter. With that said have you tried using something like a really small air hammer or needle scaler to release the lifter? Have you tried it with it at the top and the bottom of the cam lobe?
As far as that so-called crack, I've never seen one actually cracked. Is it possible that is just a casting seam from the casting sand they used in the manufacturing process. I've seen that a few times before.
These lifters fail because they lacked oil pressure during activation. This could be caused from a number of issues but usually it's because the engine is overall lacking oil pressure. The magic number for this pressure is 26 psi. If you want more information I have a copy and paste thing I could attach this that gives a detailed information about this. And yes the copy and paste is because it is a bit redundant sometimes haha.
You mentioned using something like ATF because it's coming up, well it's not actually gummed up. Your engine appears to be gummed up but the lifter is going to be damaged internally from the metal binding on itself. I have yet to pull one of these lifters apart that is stuck because it's gummed up.
If you look at the valley plate picture towards the left you can see a crack in the metal... That is the section that works cylinder 6... Would that directly affect it as well?