I ordered the spring and put the tension back together. Trouble is, the pin seems to be ever so slightly bent, so even after cleaning and buffing, it's not reliable, resulting in the tension not engaging when I put the needle down. Back to the ordering board. Considering how chicken I was to work on the tension, my reluctance just seems silly after all this. I do believe I could dismantle the 15-91's bobbin area to fix the tendency to hang up the occasional loop.
Ah, its the adventure.