Comparing how much my other rifles catch the buffer spring retaining pin, this doesn't seem like much. Is this enough?

Looks like it might go one more turn. You'll know if it goes. It won't if it don't.

That's as far as it goes.

Even while pushing down on the plunger tit? If so, then that's it.

I agree it doesn't look like much engagement. Hard to tell from the picture.

Even pushing down the plunger as far as I can, I can't get another rotation out of the tube. I suppose I could gently file away some of the lip on the tube end, allowing me to get another rotation. Good idea? Or am I asking for trouble?

According to what you say, it's good where it is then. Remember it's only holding that plunger from flying out. No need to file it.

ETA: I blew up the picture on my monitor. It looks right.

Sounds good. Thanks!

I agree, never file anything. Well of course unless something is not made correctly and you have no other choice. My example is that a lower I was building, did not have the bolt hold open area cut/formed right from the factory-(on an Anderson lower it was a common issue,I had 2), so the lever would not fit in place.I had to file down the area, some might file the part, but on a part that gets a lot of punishment/sudden forceful impact, I figured it would be better to file the area with too much material.

It's fine. Once the upper is on, the buffer is against the BCG and relieved pressure off of the pin.