Can I attach a rear cargo rack directly to the seat clamp?

If the adjustments will allow, I would use the original clamp for the seatpost and the new add-on just for the rack. By using the new clamp just for the rack you can move it into any position to make the rack level including mounting the clamp upside down if need be to allow clearance for the seat quick release. You may want to limit the loads as most seatpost mounted racks have a load limit of about 25lbs.

I’d try it, unless you need high reliability (e.g. because you are traveling in remote areas). If the welds break you can still get the aftermarket clamp.

There is little load on the top attachment points of a rack. Most load is borne by the bottom attachment points.

Before dual eyelets on the seatstays, using a single strut going to the seatstay bridge was common. Most racks either have the holes in the center for this or can be drilled. Then you flatten out a twisted strut with a vise or pliers and put a 90 degree bend in it at the end. Fine for all but heavy loads and much neater. And presuming this is some kind of racy 90s GT hardtail like it looks, it probably has short chainstays and you wouldn't really want to put a heavy load on it.

Doing what you propose could also work. It would need extra long struts on many bikes. If the struts are really long, the triangulation from having two of them might yield little or no stability advantage over having just one running to the bridge, which will be much closer. I would do that last before the above or just running p-clamps on the stays.

Using this area as the upper strut attacment point is good to do on really small bikes when it's the shortest run for the strut(s). That's when anything else would be a contrivance. So that's the good application of the rack-mount collars or of doing what you propose.

No - the seatpost binder bolt is way too high for the design of most racks, and it is only a single point.

The common solution is some stout P clips around the seat stays, one per side, ideally with a rubber backer, and these are bolted firmly to the nose of the rack. These should be just above the brake bridge, so there's something to stop them drifting down the seat stays over time.

This solution requires the two stays on the rack to run down to the rear hub area, because they do most of the work supporting your stuff.

While you can mount the rack to a brake bolt, that's only 3 points of contact total, and not enough to prevent twisting forces.

If you can't find these clips, I made some out of strips of aluminium, which worked perfectly for years.