You bring up some very important points to consider.
I might add that when taking vitamin C, it is usually considered more effective to take several small doses rather than a couple of large ones.
For example, if a person was trying to take 4000 mg of vitamin C daily, they would be better off taking 250 mg each hour for 16 hours, or 500 mg each hour for 8 hours, than taking 2000 mg in the morning and 2000 mg in the evening.
A person taking both vitamin C and Miracle-Mineral-Supplement needs to pay attention to the schedule they are on. You may be correct in that a person may have to choose one or the other, or they may find better results by alternating days.