Garmin Fenix 6 Pro catastrophic update

Did you also copy all the files from the 16.10 beta to the watch, and made sure to put them in the right folders ? On the beta page below is the description.
If that does not help, I would take the backdate file, and downgrade to the latest public version. If that doesn't help, I would install the same version again with the webupdater.

I did it accordingly to description, all but one file (main upgrade) are in same folder (remotesw) - and funny thing is that all are consumed by update, apart from sensor hub, which stays there. 
I've tried via webupdater, but with version 16.10 on board, I will try with downgraded one.

Which FW version did the autoupdate install for you?

BlackWebWolf said:

I've tried via webupdater, but with version 16.10 on board, I will try with downgraded one.

16.10 is a beta version. The webupdater only works with public versions.

yes, I downgraded to 15 (last public) - and still it installs same FW version, 8.0 for Sensor Hub

Have you tried a secondary master reset with buttons (as described here https://support.garmin.com/en-GB/?faq=wxPmdnWsev1xGRUf58oDC8)?  That secondary master reset clears all data, and not just the settings, and is as close to a factory reset as any of us can do.  Following that, do the most basic of setup steps on the watch so that you can then attempt reinstallation of all of the 16.10 files (or use the other files in the beta backdate software package to go back to 15.20 - ensure all of the respective component files are are correct too).

Through all of the above, do make sure you are downloading the correct software 16.10 package for your watch (you don't specify if your have a 6S, 6 or 6X and it does matter if they are pro or non pro variants as there are different softwares between them too.  If you are unsure about pro or non pro (sapphire variants are all pro), plug in to your pc/lap[top: any watch with multi GB of storage for music/maps is the Pro variant, any watch with tens of MB of storage is a non pro variant)

If all of that fails, then I suggest opening a support case with Garmin support in your country/region, as it may need RMA or some special software files to force an install that only they can provide (it's their problem to fix, as this happened when you were installing their routine software update).

BlackWebWolf said:

BMX: 0.0.0 (!)

This bit is odd too (the exclamation mark), and has been an indicator of an incompatible peripheral software for the main system software version (usually seen if you install peripheral software that is too new for a current old system software).

Confusion intensifies - after mentioned master reset I was welcomed by... update file in remotesw (removed just before reset) :D I must say that it is a really interesting way of confusing people :D 

What is more - in evntlogs directory there are nice entries in logfiles - like `HWM misc  Update Error. Time between updates: (range from 0 to 20317823) ms`
In general - thanks a lot for help, but looks like RMA incoming. 

So far any luck?

I had similar problem, but on the GPS, it shows 0.00 after 15.20 update via garmin express. move to 16.10 and back to 15.20 does not help, still showing 0.00 for GPS. To make matter worse, there is no support and warranty in the location I'm at.

Yes any news here?