...it's going to have to be surgery.
I met again with the doctor this afternoon, and he managed to both explain everything to my satisfaction as well as show me beyond a doubt why a procedure was necessary. Even though I'd been feeling better, and could raise my leg quite a bit more than after the accident, it turns out that I wasn't getting the complete range of motion; that final five degrees wasn't there, and I could see that when he had me raise both legs side-by-side.
So the second MRI was indeed telling the truth; the tendon isn't completely ruptured, but it's only hanging on by, in his words, "fibers." The reason I've been able to get the range of motion that I have so far is because other nearby muscles have been overcompensating for the unusable tendon, but that's not a long-term solution. What I was told was that, if I were to stumble again in any fashion, my entire knee attachment might well fall apart, and that doesn't sound good in any way.
I've asked to have one more week to teach before this happens, so that I can see everyone through tryouts (and witness a very cool event that I'll talk about as it gets closer), so the procedure will likely be right after Memorial Day. I'll update things as I know more.