Appreciate projected 2015's usable 50% range from loss of first health bar at 48,000, and now 56,200 with 11 of 12 bars?

I'm curious on getting an estimate on remaining useful battery life from the following data points:

• 1st health bar lost at 48,000 miles
• Still has 11 of 12 health bars at 56,200 miles.
• Vehicle maintained by Nissan, so perhaps has latest firmware. How to find out?
• 2015 Nissan Leaf, bought as a 2 year lease turn-in with only 15,000 miles.
• Located in cool Colorado Springs, CO at 6500' elevation. Rare to get over 90 deg F.
• Don't have Leaf Spy software app to check health (perhaps should?)
• Only very rare fast charges from dealer Chademo with degraded power modules putting out only about 18 kW
• Cell balancing about every other month or less ... 90 to 120+ minutes past 100% on ChargePoint to observe zig-zag W's
• Only other times past 80% is to about 85% to 90% and then within minutes driving uphill back to home.
• Rarely drain past 20%
• Probably 50% of juice from home Level-1 120v, and remaining from ChargePoint and other free Level-2 chargers.

Is it reasonable to guesstimate getting about 150,000 to 200,000 miles with about 50% of original range remaining?

You should get Leaf Spy so you have a battery idea of how close you are to losing the next bars.

I don't know of any new firmware for a '15 Leaf that would affect battery capacity or its reporting. From doing a quick Google search for average weather of your city, you live in a pretty good climate in terms of battery degradation. The # of miles when each bar went away isn't that helpful/important. What date did that happen?

At least you have the "lizard" pack which is the best of the 24 kWh packs.

The main variables that affect battery degradation are
1. Time at higher(er) battery temperature AND high SoC
2. Time at high(er) battery temperature
3. Time at high SoC

Notice that distance traveled is close to irrelevant. Second, you do not know the history of the car with its original owner or where in the 12th bar the car battery was when you took possession. You also don't know how much of the 11th bar is used up. So for now you lack the data to guess at future degradation.
The first capacity bar drops when ~ 16% of the battery capacity is lost. Your next bars will drop with a loss of ~ 6% capacity each. When the 11th bar drops you'll know how long that took and you can extrapolate from there. The top of the 10th bar is ~ 77% capacity remaining so 27% to go to get to 50% remaining capacity, meaning a little over 4 periods of 6% drop each.

It sounds like you are taking good care of the car battery. The only additional things I do that you did not mention are
1. The car is not parked on hot asphalt
2. I charge in the early mornings in the summer, and after driving in the winter

I think we live in similar SW high desert climates so here is my LEAF's degradation history. View the last year with some skepticism since the car has not been used much and I'm not sure that the BMS is well calibrated. Ahr is another way to express battery capacity. Our "24 kWh" LEAFs started out new with ~ 64 kWh