why engine locked up when engine was shut off at end of short drive and after the only apparent change while driving was to change to second fuel tank which is suspected of containing old gas

Pull the plugs and put a big socket on the crank and see if it is really seized up or not.

yes pull handle on crank will not move it with plugs out and starter off

You could use a bore scope to inspect the cylinders but it sounds like it is toast.

does that suggest it can be saved by top end fuel /air chambers cleaned of contaminants and cylinders loosened with solution/deoxidizer ? possibly?

That sounds like the pistons are stuck to the cylinders which means it needs a full rebuild.

and the old fuel would definately cause this?

Who knows what was in that gas tank? This does not sound like a coincidence.

Nothing in the gas tank can cause an engine to seize. You either lost oil pressure, or bearing shredded, you won't really know unless you do a tear down on the motor.