I just checked availability for accessible inside, oceanview and balcony cabins. There's good availability including three balcony cabins on deck 7. It may be that RCCL had the cabins temporarily blocked for some reason when your TA was checking.
Because your dad may not be comfortable on his feet for long periods of time, they might want to consider the accessible cabin because of the grab bars by the toilet and in the shower, the larger shower and the shower stool.
Whether or not they use an accessible cabin, suggest to your mom that they take a heavy duty extension cord. Also, they need to be sure they and their TA are on top of all requirements and details pertaining to the O2.
I've sailed on Enchantment twice and, like Nancy, have never seen an excessive wait for the elevator. But I do think deck 7 might be slightly more convenient for them.