I haven't got a T420 but an L430 with Broadcom bluetooth hardware. On those devices you typically need to install a firmware file to make the bluetooth module work. Have a look at -bt-firmware and the instructions shown there. Typically it's as simple as copying the right file to the right location. As far as I remember, the file can always be extracted from the Windows drivers provided by Lenovo.
OK, that probably means the required firmware is in place. I would have expected something like "Direct firmware load for ... failed with error -2" otherwise. I just looked at the output on my L430 and it is pretty similar. Sorry, I can't really help here.
I also remembered that my laptop was shipped with a small drive with Windows 10. I popped that in and the bluetooth also didn't work. The bluetooth daughter couldn't see any devices, Windows didn't recognize it at all. The dongle, however, could see my bluetooth mouse, but couldn't connect to it. It's a mystery...
The external dongle is listed correctly under lsusb. Sorry, I expected the internal bluetooth module to be some kind of PCI device but it is not. I checked on my L430: The device shows up under lsusb (not lspci!) but only after it has been activated.
Can you check if you have the thinkpad_acpi module loaded? That one seems to be responsible for activating/deactivating the bluetooth hardware on my device. If not, try to load it manually. What does "dmesggrep thinkpad" give you?
Thanks for your extensive research, Bevan! Much appreciated. Unfortunately, I have neither the equipment nor the experience for SMD soldering, but thanks for the link nonetheless! It's reassuring to hear that I am not alone and the faulty fuse is quite plausible...
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