I do hope that Google's supposed changes will enable the Nexus 5 for VoLTE. Even after all this time, it would actually help the industry (as well as enabling any other devices that are capable) because everyone needs more people using VoLTE.
I still have one that's going on strong (like the Nexus 7 2013, these devices seem so much faster than more modern devices with custom skins, customisations etc).
When it comes to what the Three coverage checker says, it seems like they let people play around with the settings all the time - hence stuff popping up that isn't strictly accurate because it hasn't happened yet.
Not only would it make sense to sort the tablet/data-only products situation, but Three might also want to advertise the fact. It must remember that it sells mobile enabled tablets and sells mobile broadband SIMs, so besides going on about 'super voice', why isn't Three pointing out the hugely improved coverage to boost MBB sales?
Oh, I forget, Three has little to no interest in business customers and going by the plans, little interest in mobile broadband at all.