Hi, if I remember correctly, you are young, under 30? If that's the case, extremely dense breast tissue is almost a given. Also, if I'm correct (I may not be) you'd probably be covered for MRI of the remaining breast. I'm actually kind of surprised that it wasn't offered previously, but that may be because you went straight to mastectomy.
I think ask the question. If it comes down to it, you could always have chemo next, and radiotherapy if recommended and give yourself time to think about what to do about your other breast. As @kmakm says, trust your gut. It is, however, also important to trust your team.
Throughout this crazy path we've all found ourselves on, we have to learn to be our own best advocate, and that can be hard, too