B) Forscan just has an estimated battery capacity remaining to empty. Alternatively there is an "enhanced" trip meter that says how many kWh/ have been used from the battery. I have used this with a little math to estimate my usable capacity. For example full charge down to 10% battery remaining would be 90% of my usable capacity. If the enhanced trip meter says I used 15kWh to get down to 10% left, my estimated total capacity would be 15kWh/0.9=16.67 kWh total capacity.
Below is a screen grab showing my FFE at nearly full charge last winter. About half way down you will see "Hybrid/EV Battery Energy to Empty - Estimated" Mine was at 16.2 kWh. In summer as the battery warms up the capacity goes up. IIRC my battery capacity is a little more than 1 kWh more capacity in summer. It started out at 19 kWh when new.
These parameters are under the BECM module.
C) The short answer is it's very gradual, akin to just having a few more charge cycles on it that it actually does. I don't know that anyone has any hard data on how calendar age affects the FFE battery. I would guess it's maybe 1% to 2% per year and probably isn't linear? From battery degradation reported on Tesla's you lose up to 5% fairly quickly and then the degradation is very gradual. Not sure if this translates well to the FFE battery or not and this data is for ALL degradation not just calendar age. In my 5 years of ownership I've only lost about 5-10 miles of range, although some of that may be due to the fact I don't try to drive efficiently as much as when I first got the car.
D) If you need to go 50 miles daily, I hope it doesn't get cold where you live. Winter weather can sap up to 40% of your range.
Also you better have good LRR tires. Normal tires can sap 10+ miles of range.
Also depends on how you get there. An EV is the opposite of an ICE vehicle. You don't get as far on the expressway as you do in town. If you think you're going to drive 80 mph and get 60 miles of range you will be greatly disappointed.
I"m not trying to talk you out of it, just trying to give you realistic expectations.