why are 2nd and 3rd shots of group lower

[FearNoWind]

Could the rear block not resting firmly on the stock explain either the inconsistent group sizes or why the first shot was always high?

Yes, of course.
Anything that allows movement (forward, to the rear, sideways, up, down) is going to affect accuracy. I'd never try to hunt with a rifle that I couldn't depend on and "waging" it with adjustments that provide you with the least amount of error isn't the same as accurizing your rifle.
Until you bed that rifle you'll never be able to asses its potential and you'll never be able to rely on its accuracy.

 

[dmax1800]

Update.
I couldn't find a gunsmith on such short notice to do a bedding job, so I got a kit and did it myself.

Took the rifle to the range to test it. Group sizes are still a little sensitive to torque amounts on the action screws, but no where nearly as sensitive as they were. Changes in action screw torque didn't have much effect on POI.

It was too windy to shoot at 100 yards, so I shot at 50. Yes, I know that 100 yards would have been a better test, but I figured that the wind would be too much a factor (15 to 20 mph). One group was 1/4" and the other was 3/8", so that makes it 1/2" to 3/4" at 100 everything else being equal and assuming I can hold the gun steady enough. Not too bad for a belted magnum.

All in all, the bedding job helped quite a bit. Many thanks to everyone who gave their input on this.