Would lack of ring gear cause the starter to lock up during attempted start?

No. The starter would spin as it was not engaging against the missing ring gear. (The telltale heartbreaking sound of a missing ring gear is a "whhhiiiiiizzzzzzzzzzz" as your starter happily spins with no load). However, your starter issue could be related to a drained battery not supplying nominal voltage. Very common when the car has seen downtime, or when the leads were not tightly reconnected. Have you put a voltmeter on your battery?

Not a batt issue- can't turn it from the fan pulley.

will it move at all on the pulley? even just a little?

I had an experience similiar to this with my 912E (915 Trans same as yours). I installed a new clutch during my engine rebuild, and somehow the clutch hub ended up touching the shaft housing of the transmission. The engine ran for about 3 seconds, and welded the clutch hub to the trans shaft housing. Drove me crazy trying to figure out what was going on.

Try pulling the engine from the trans. If it won't budge you probably have the same problem. I had to remove the starter, to get access to the pressure plate bolts, and have someone hold in the clutch while I just barely was able to rotate the engine with a big lever, and remove the pressure plate bolts.

Hope that's not the problem, but it's something to consider. Good luck.

You might try turning the pulley backwards, or CCW just a fraction. If it wants to move, then you were most likely at the point where athe bolt mentioned above made contact.

What's interesting here is the 50 degree turning and then resistance comment in your first post. Obviously, you should have investigated further at that point, but you're beyond that now. I hope there's no coins, nuts or small tools in your distrubutor bore. You wouldn't have been able to reinstall your dist, but something may have fallen a bit further in. And what did you mean by your boy "watering down" the motor? Did he pee in the hole? LOL :D

Watered the engine....
I had one engine that someone had stuck a sparkplug in the breather hole.
The one in Blackbird now I bought on Ebay. It arrived able to turn only a few degrees and then a good hard CLICK and no movement. Turn it backwards (uh-oh, what does that do?) and the same after about 60 degrees. It had a ball bearing in #3 that I was able to get out by hanging the engine from the garage rafter on it side and rinsing out the cylider with denatured alcohol. The PO has kids, so do I. Which one did it doesn't matter, I just wonder if he really didn't know about it.. .

Watered the engine....
I had one engine that someone had stuck a sparkplug in the breather hole.
The one in Blackbird now I bought on Ebay. It arrived able to turn only a few degrees and then a good hard CLICK and no movement. Turn it backwards (uh-oh, what does that do?) and the same after about 60 degrees. It had a ball bearing in #3 that I was able to get out by hanging the engine from the garage rafter on it side and rinsing out the cylider with denatured alcohol. The PO has kids, so do I. Which one did it doesn't matter, I just wonder if he really didn't know about it.. .

FIXED!!!!
 

See new thread- I found the miserable little culprit. It was called a shroud bold used in the pressure plate. ..