I’m so sorry that you have to deal with the mammogram imaging of your armpit. Whomever designed the mammogram device put no thought into how it must fit against a human body - which is definitely not square with straight edges.
One mammo technician was telling me (when she was pulling skin and I was practically moaning) that she does this for my own good (true) and that in one case she was able to include enough under breast skin that cancer was found in that area. I can only guess that she pulled hard enough that lung cancer was detected 🙂

Now that I go to a cancer center, they often schedule the mammogram before the MRI, not sure it that’s intentional. But in your case, if they could do the MRI first and know whether there is something that they need to see on the mammogram. Maybe they don’t need to image the armpit area this time, if they don’t see something on the MRI.