After many design changes and circuit complexity, I've decided to replace the under-rated Microtemp brand thermal fuse ( unavailable in Canada) with a 25A mini blade fuse. The original fuse leads are less than a millimeter wide, and couldn't tolerate the fan current. I'll install thicker wire leads from my stock, bending them to accept a 3mm female crimp connector, and bend the fuse leads inside the shield. This should, in theory, keep the mini blade fuse from melting and give me all four intended fan speeds, not just full blast through a relay. The wirewound resistors kept their forms and didn't burn up and break from their posts.
I'll keep the proposed idea for a future motor project.