"The bolt that goes into the end of the crankshaft has come loose and caused the lower timing belt sprocket to stop turning under load - that's why the valvetrain turned when I cranked the starter."

That doesn't sound quite right to me. If the lower (crank) sprocket stops turning, there is no drive to the belt. So how could the "valvetrain" (camshaft) turn when cranking?

More importantly, that crank pulley bolt doesn't just "come loose" unless it wasn't tightened properly (44 ft lbs, then an additional 60° of angle tightening).*

I see by your post below that you'll be replacing the pulley, in which case I strongly suggest you use the ROPE TRICK as a counterhold when you tighten the pulley bolt this time.

* When I angle-tighten tighten that bolt to spec, I have to use a pipe extension on my 18" breaker bar, and pull with both arms while braced against the left fender.

Here's wishing you a speedy recovery from your injuries.