Success at last! I completed my upgrade using the basic adapter from China and soldering D+, D-, and +3.3v as mhafeez illustrated. In my testing, trying without the +3.3v, the bluetooth shut off with the wifi as expected. Like mhafeez, I did not cut a power trace, but by adding the bluetooth power as he showed, I now have bluetooth when wifi is shut off. And incidentally, the wifi functionality and control does not seem to be affected by having the wifi power bridged (trace not cut) with bluetooth power.
I reused the bluetooth antenna by purchasing the 24" extension as noted by scroberts72 in a previous post. My mouse works fine with this setup with no jitter that rcf362b noted.
I found that this inexpensive adapter seemed to fit better, engage better, in the slot than the osxwifi adapter did (which I destroyed trying to solder onto.) And I was able to reuse the original wifi mounting screw with this adapter.
Upon startup, my iMac recognized the correct bluetooth hardware, as others have stated, but I could not get continuity to function. I tried restarting both the iMac and iPhone 6 and I tried turning the settings off and on. I tried using the Dokterdok Continuity Tool with no success. It finally started working after I logged out of iCloud on both devices, logged back in, and then restarted the iMac again. That seemed to do the trick.
Thanks to all who have contributed to this forum.