While I sympathize, there are reasons some of this takes place. What if on a macro level, anyone could just order a B12 test? You would likely have lots of unnecessary tests driving up costs for everyone. So insurance shouldn't necessarily be faulted for that. Now the $25 per visit does suck, so you would think they could come up with some kind of "package deal" for everything. Sorry :/
One shortcut for your situation: why not just ask for the shot and ask the doctor to forgo the lab test. Getting a shot of B12 isn't going to hurt if you're not B12 deficient. If the symptoms persist, you're going to go to the the follow-up appointment anyway, and it can be addressed with the lab test then. That may or may not be possible, but it's a thought.
It does seem like a virtual appointment (or just a phone call) might fit your situation if she is asymptomatic after the shot.