Why won't my engine rotate?

After doing some maintenance on a 2000 Porsche 911 engine (3.4l normally aspirated), I find that I can manually rotate it with a wrench on the flywheel, to a certain point but no farther and I can’t figure out why it won’t rotate.

The details

I removed the engine from the car and replaced the rear main seal and the intermediate shaft bearing, which may have been seeping some oil and is a notorious weak spot for these engines. The intermediate shaft bearing replacement was done using a procedure recommended on Pelican Parts. (The linked procedure is for a Boxster rather than a 911, but the procedure is essentially the same.) The chain sprocket behind the bearing is held in place temporarily by set screws to prevent it from rotating and messing up timing. Then the bearing is replaced and the set screws removed. I did all of that and then decided to recheck the timing to make sure everything was still OK. It wasn’t.

With all 6 spark plugs removed, I was able to rotate the engine through about 150 degrees or so, and then it won’t turn any farther. I thought that perhaps I must have made some error and that somehow the timing was slipped (but note that I don’t know that this is the case or not) so I used a borescope to look inside each cylinder. However, while some valves are open, there is still clearance between every piston top and every valve bottom.

This diagram shows the orientations of the relevant pieces although this diagram is a slightly later version of my engine (mine has 5 chains; this one has 3). The intermediate shaft is below the crank shaft and the intermediate shaft bearing is not quite visible in this picture. However it’s easy to visualize where it is — it’s at the far end (top of picture) of the intermediate shaft, just below the main crankshaft bearing.

So my question is, why won’t my engine turn?

Additional notes

Here are some additional notes that may help better convey essential details I inadvertently left out of the original description.

1. I would turn it to TDC to check static timing but that’s what I was attempting to do when I encountered this unexpected problem.
2. The cam-locking procedure step that was inserted as a note into the linked procedure wasn’t in the procedure I followed (which was in a printed book and not online), so the cams were not locked prior to this prepare and are not locked now.
3. The cams can’t be locked now because the tool only works when the engine is at TDC, and see point #1.
4. I do not want to risk further possible damage by rotating the engine backwards.
5. Based on measurements of the angles of the camshaft ends (which are visible since I’ve removed the protective caps), it appears to me that if the timing is off, it’s not off by much.
6. The engine is currently on an engine stand with the spark plugs and transmission removed.
7. I’m willing and able to undertake the additional effort of removing the exhaust and cam covers, but would absolutely rather avoid it until I can first make some convincing diagnosis that requires it.
8. I have the full sixteen volume Porsche shop manual for this vehicle.
9. I’d really rather be driving. 🙁