Ok I'm not an expert but before you give up on the gun lets try a couple things. Recent S&W's have a new style rachet star. The old one had two pins that line up the rachet. The new style doesn't have the pins it needs the shells in the cylinder to locate the rachet. If yours doesn't have pins in it you need a clip with empty shells in it to accurately check timing. If yours doesn't have pins notice that without shells in it the star has a little bit of slop, it needs shell in the chambers to line everything up. The new style rachet will give the appearance(sometimes) of very late timing without shells in the chambers. I noticed you didn't have emptys in the gun in the video.
I have 2 older 625's that I shoot in competition and both have many thousands of rounds thru them, they have pinned rachets and time out excellent. I watched your video and went and checked mine and if I hold gun cylinder release down like you did in video and put a thumb on the cylinder like you did I can make mine do the same thing(cylinder can be rotated clockwise) as it doesn't reach lock up, both guns do this but when you hold them without putting thumb on the cylinder they time way ahead of the hammer drop. As I said if you have a non-pinned rachet star that may be why it shows late timing with the cylinder empty and no thumb on the cylinder.
Also if you champer the the cylinder for faster reloads and you have a non-pinned rachet REMOVE the rachet star and champer the cylinder by its self to the depth you want. Then champer the rachet by its self. When you do the rachet make sure you leave some of the sidewall(the part that matches the chambers) for proper timing. The cylinder can be champered alot deeper than the rachet. Pinned rachet stars can be champered with the cylinder as the rachet being pinned timing won't be affected.
Hope this helps.

Last note if I understood your video gun is shooting low with rear sight all the way down? If so rear sight needs to come UP to raise point of impact.
Sorry if I misunderstood.

Sorry about the scratches and stuff, unfortunately buying a gun unseen from so far away has its risks. But S&W needs to get their act together, especially for a PC gun.
Good luck