Something very odd is going on here. Now nothing works with the following three files in /Library/Extensions:
BrcmBluetoothInjector.kext
BrcmFirmwareData.kext
BrcmPatchRAM3.kext
From an article by RehabMan in 2015, I concluded that this combo should work. However, if I look at the info.plist inside of BrcmFirmwareData.kext, I see it was written for MacOS 10.14 (Mojave). Is there an update for 10.15 somewhere?
The System Info readouts (jpgs zipped below) show all the good stuff for hardware except that the firmware version number for the BCM94352 card we are using shows "v14 c4096,"and RehabMan wrote that if the number "4096" appears, it means the firmware has not been updated on the card. So apparently these three files, where they are, just don't do the job. Other Bluetooth devices (non-Apple) apparently work fine, and I did see the Magic Keyboard connected wirelessly and working one time, with all three files installed in both locations, as I wrote above. But no more.
If anyone has figured this out (I will try Feartech's placement next) I would greatly appreciate a working answer to this.