HRM-PRO Autocalibration

It might not be clear indeed.

But in the same time just because option is "grayed out" it doesn't mean it is not autocalibrating. Maybe when you set "Always" it is always autocalibrating and using both watch and HRM pace. It is just un interface. How it works behind the scenes that's only dev's know ;) and yeah some clearer explanations would be nice

No, in the FAQ Garmin specifically stated that one of ways to turn OFF the auto calibration is to switch to "Always ON". I am too little surprised that they configured it that way, especially that they enabled auto switching between GPS and HRM PACE automatically, depending on the strength of the GPS signal.

For 1 and 2 just ensure you run outside for around 2 hours. The time is not crucial but the more you do outside the better the calibration. There is know way of knowing otherwise.

For 3 and 4 the answers are no. The information is on the strap.

Will it be retained in the memory of the strap if you need to change the battery?

Not sure about that one. 

I am after autocalibration 2.5h.
GPS+Galileo worked better than HRMPRO.
I write about it in another post

I can confirm that with the strap set to "always" it will default to that for pace and distance instead of the watch when outdoors. I did an intervals session a few days back with a strap paired to my epix and every split ran long and came in 10-12 secs slower than my usual times, plus the overall distance was roughly 400m short on my course which measures close to 6.6km normally. Live average pace on the watch read on the low side also. My kids were wearing their venu sq whilst following on their bikes and the distance was spot on to what I finish at normally.

It means this fake option in hr to improve speed and does not works like stryd or footpod

What about using it only for pace and gps for distance?