I leased a 2014 for 3 years and reupped for 2 years on 2017. Gotta love the big cabin feel in a small footprint. 22K in 5 years is about right for regular but not excessive use.
To your question.
1. The indicated range is a guess like in a gas car, depends on last miles driven. You may want to reset the indicator and compare charge level against reset range. It is a very rough indicator of battery health.
2. Best is to take the car to a MB dealer who can run out battery report and check car out for you. The battery test is a annual requirement and if they know what they are doing (questionable), they can access the past reports online.
3. You need to find a local MB dealer who can service the car. Stuff will happen (per posts) and if there is any remaining warranty on the car and it ports to you, you still need a place to have it dealt with.
4, You didn't find much here because as far as I can remember, severely degraded battery has not surfaced in these cars yet. They are Tesla batteries and motor and others have found useful info on other car forums.
NOW TO THE CENTRAL QUESTION OF RANGE. YOU ARE FREE TO READ THE POSTS OF COURSE.
Not all 2014s had the option to top up the battery to 90 percent of capacity. GlennD has posts that state that MB sets max charge of batteries at 80 percent (but translates to 100% in car indicator) Confused yet?
The option gives 12.5% more range and I use it all the time, particularly when the outside temps drop below 50 degrees.
You will want to consider daily use, places to charge, and if you are so fortunate, a dedicated plug in your warm garage.
I clearly get by even though I live in an apartment building and park on the street. It is something you are well advised to work out before you buy or lease to anticipate the dreaded "range anxiety" attacks. Not recommended but I have driven both this car and gas cars down to 0 range and managed to make it to a fill up location.
Jeff in NYC
MB '17 B250e; MB '06 E500W; MB '81 380SL