I doubt there's any missing spacer, the cassette is usually mounted as close to the frame as possible while allowing chain clearance. You can check the chainline quickly by laying a straight edge on a secant edgewise against the face of the outer chainring and carrying the line back. It should meet the cassette outside of centyer by an amount roughly equal the the distance from outer face to center of the gap on your chainring.

In any case, it isn't a matter of inline or not, but of degree. Any chainring can only line up with one sprocket, and with normal chainline the inner lines up just inside the center of the cassette, and the outer just outside center. After that all the sprockets are off line to a degree, getting worse as you move to the ends of the cassette. Cross chaining makes it that much worse.

BTW- note that in your video, you're running it backward, and the chain is actually coming off the sprockets in the real world. the engagement at the bottom is always smoother because the RD is feeding it in line.

If you read various threads you'll note that some systems including some of the most expensive have a reputation for being noisier than others. The key is the amount of bevel of the teeth, the amount of bevel on the inside plates, and the lube.

Don't look for a problem that you don't have, focus on making adaptations to address exactly what is happening. As i said it's normal and to a degree unavoidable.

Looking at your video it seems that your chain has very square plates, Many others have more bevel which lets the chain off easier, ramping off the teeth, rather than jumping off as they disengage. I suggest you try a new chain looking specifically for the model with the most bevel.

Also since the issue is worst on the innermost sprocket, try grinding a shallow bevel on the inside of the teeth, either freehand on a bench grinder, or with a Dremel, while turning the cassette (like a lathe).

You'll find that either or both in combination will make the drive train much smoother.

If you want to see how much difference the oil can make email me at my site below, and I'll send you a sample.