Well, I got the phone call back from the doctor ("Dr. R") today. He spoke at length with the surgeon ("Dr. K"), and Dr. K did not believe an MRI would give him any more information than he already has.
The only option other than waiting is to attempt to tag the site. The plan would be (if I have this correct) to inject a metal marker into the site and biopsy it at the same time. Then follow up with another mammogram to make sure they got it. If not, then it's "second verse, same as the first" until they do get it.
I don't like waiting... Two days was enough for me. So, even though Dr. K wasn't confident he'd be able to find it with the needle, I'm going to call his office and make the appointments. I guess I'd rather just get it over with and know already, even if this takes multiple tries.
Why the change of heart? Mostly, friends who have gone through this experience, either first- or second-hand, telling me not to wait. Friends online, at work, old friends, recent friends... people are coming out of the woodwork with their personal stories. The common theme of every one: don't wait. Get as much information as you can, and keep going. I'm not good at leaving things unanswered anyway, but this was the most compelling reason to keep pushing. Thanks, everyone.
I'm planning on going out for multiple beers after this is over. Anyone with me? :-)
Update: I have an appointment with Dr. K on 2/18. We'll see where this goes from there!