SUCCESS!!
After 2 months of (moderate) misery I've finally found the reason SMB didn't work with my Sierra Mac Mini.
TL;DR The reason was this installed kext:
com.intel.kext.intelhaxm (6.0.1) 8FF2C637-0A5E-367E-B007-5B08655B1E8A <7 5 4 3 1>
You can check if you have it installed with the following command from an ordinary Terminal:
kextstat | grep -iv apple
In case you have it (and in case you're also suffering SMB connectivity problems) you can uninstall it typing the following command, again, from an ordinary Terminal (no need to boot in Single-User mode):
sudo /Library/Extensions/intelhaxm.kext/Contents/Resources/uninstall.sh
Follow the on-screen instructions, REBOOT, and you're done :)
Doing that you'll lose hardware acceleration inside your Android emulators, but they will work in software-rendering mode. Not brilliant, but it's something. You can reinstall HAXM in case you really need Hardware Acceleration for Android emulators again (but be prepared to lose SMB connectivity again (?)).
Long answer:
If you use your Sierra Mac to develop Android stuff you'll probably have installed the typical random needed modules (Android SDK's, emulators, drivers, etc...) The thing is, "Intel HAXM accelerator" is one of the typical drivers you install if you want proper hardware acceleration of your emulators for Android developing. Well, apparently, and believe it or not, that driver is not compatible with using SMB under macOS Sierra, at least with my MAC.
Sierra SMB subsystem and HAXM are apparently unrelated pieces of software, but it seems they are somehow incompatible between them. In case you have SMB problems you'll have to decide which of the two you really need more:
SMB or proper fast emulators for Android development.
I chose SMB :)
Thanks EVERYONE in this question, answering, commenting, etc... specially Brett who, after many weeks, put me after the correct lead.
