Heard from Nala's vet today that her blood work results came back fine. No increase in circulating mast cells, which is good. However there was an incidental finding of slightly decreased thyroid hormone levels which would certainly explain why Nala's such a mellow, relaxed dog and why she likes to sleep all the time.
The lump has shrunk considerably since starting her on Benadryl. Tuesday is surgery day where the lump will be removed and hopefully that will be the end of Nala's MCT problems. I'll have to make sure to give her a good going over every now and then to make sure no others pop up though.
Thanks to everyone for their well wishes for Nala.