So the warranty company is saying it's an emissions issue but BMW NA is saying it's a maintenance issue? Very conflicting.
Honestly, I think your only option is to take the warranty company to Small Claims court and report them to the Better Business Bearu. Read the fine print of the warranty; see if it says specifically injectors are not covered.
If BMW is willing to agree its an emissions issue, I believe by law they have to cover it to 75,000 miles. All emissions related components are supposed to be covered till 75k. Do some research.
I would just pay now to get the car and fight later. I think you have a good chance of winning if you document everything properly.