I had a 2014 and the battery was checked under warranty each year. Who told you this was not so? The dealer can access their system to confirm if it was or was not done and what the readings are. The battery after 1200 miles is barely used, what is the story with that?
What does Hard/No Start mean? If it has any warranty left, this would be covered. Make them do it before you buy it. If you are in snow country, would recommend you get one with the range extender option.
Frankly all three have low mileage so I would go for door number 1 and negotiate the price down to $23ish. The 2017 has the range extender as standard.
On the two 2014's, I asked the MB dealership selling them can they confirm the battery warranty was still valid in writing. Specifically, I pointed out the section in the battery warranty fine print about the annual service. Sadly, they would not put it in writing as both of them missed at least one of the annual service checks. I'm willing to settle for the dealer themselves standing behind the car, but they wouldn't go that far. In the 3 months, I have been looking, I have only seen one B-class show up in the Certified Preowned sections nationally. I'm fairly convinced this is a compliance car that MB would rather just turn its back on. The 2014 - 1200 miles one is really intriguing. That thing NEVER got drove at all. However, it spooks me because how long did that car sit between charging or use? The No-Start condition listed on the Carfax is probably because someone probably let the battery go to 0%. While the charge cycles might be low on the 1200 mile one, I wonder if the battery was left neglected which also would shorten its life.
The 2017 I will need to travel to see. I told the dealer there, I want to see the service records PRIOR to 6 hours round trip in a car for me. The salesman there is a bit too reassuring that OF COURSE it's in warranty, we do the 3000 point inspection on all the cars we buy. Blah Blah Blah. End of the day, I want to confirm it for myself. I'm leaning towards the 2017 as long as I can get the right price. It's been on their lot since March so it's not moving.
Upsides - it has the 12 color ambient interior lighting package and otherwise loaded.
Downsides - No heated seats (not much of downside because there's instant heat in electrics) - White (I like blue, but I haven't seen a rich Lunar Blue B-class yet)
2014's are south seas blue, which is alright. Equally clean for their age. The dealer seems to be more willing to stand behind it for service.
My local Mercedes dealer in my burb was pretty much "You buy any B-class, and expect us to service it... I will take us weeks potentially"
I will most likely decide in the next week or so between one of the three. If the 2017 is confirmed in battery warranty, I will negotiate hard. Otherwise, the 2014s would be almost 10K before any haggling.