#1, I would say order the MRI before you see the onc. Do you want to go down one path of treatment only to find there are now other things which need to be done? Call monday to the doc who gave the PET, have him/her order the MRI and that way the onc will know everything. Given the diagnosis, the forgetfulness (which you say is unusual) and the PET results, I think to all of that at least says get the MRI out of the way ASAP. I could tell you don't like "more waiting" based on your post...a visit to the onc without the MRI already dne will probably result in more waiting.
#2 Is there someone going to these appointments with her? It is often overwhelming, esp. that first onc meeting, so I would think two sets of ears would be best.
#3 in response to your question 4, the answer is, it depends. Just go with your mom's wishes, make sure she has all the info she needs, and the rest will work itself out.
Best of luck.