If I were "D", I would pay YOU just to get rid of you!
Seriously though, this is just another example of dealers using "creative accounting" to generate business.
The battery IS covered under the original warranty. Ask them to run the Repair History on your car to see if in fact the battery was replaced by a BMW dealer.
If it was replaced by a BMW dealer, then it STILL should be covered, even if the wrong one was installed at the time.
As far as the time required for battery replacement, I would call couple other BMW dealers and ask them what is the "shop time" for this job.
"...Those who would give up their freedom in exchange for security, deserve neither!..."