I would ask the priest, but I think a better strategy would be to have the funeral Mass at the old Church as your MIL requested and bury her there, and then arrange a second memorial service for your MIL in the other area.
You may not be able to have a second funeral Mass, but the priest or deacon can definitely do a prayer service at a funeral home or other gathering place, and you can also probably have a Mass said for her soul at the church in your area, although the timing/ scheduling of that may have to be in the future if their Mass book is already full (it depends on the church).
My husband had a similar situation as he had been living and working about 500 miles away from where he grew up and where most of his family still lived. So he had a lot of friends from work, neighbors, etc. in the new area, but his family was back in the old area. I chose to have his funeral with the family back in the old area and have him buried next to his other deceased relatives in the local cemetery. This also makes it easy for his brother to visit the graves because his brother does not have a lot of money or time to travel (it’s relatively easy for me to travel there).
Then I had a separate memorial service in the area where he had been working, and his work friends and neighbors came to that, and a deacon came and did a prayer service consisting of reading scripture and saying some prayers. The deacon also arranged to have some Masses said for my husband’s soul but those are being said on various days over a couple of months when there was a space on the Mass calendar for the local church.
Perhaps some similar arrangement would work for your MIL.