Questions about Apple iPhone 11

Yes, they are authorised resellers of Apple.

In principle, there is a 6-month guarantee on the battery. Anything beyond that is goodwill on the part of the manufacturer (goodwill). A warranty extension never applies to the battery (see display breakage - here you must take out insurance because it is excluded from the warranty). I hope this helps you.

The other iPhone 11 models are available. It's probably just because the green model is not currently available from the supplier.