I say just limit it to 2 combined immunities at one time. The day after the last day would be when the person is allowed to try for immunity again. So for her it would be if there was an immunity challenge on the 14th she wouldn't be able to do it, but would be able to on the 15th if there's an immunity challenge, after a roll has been done.

Or just that there must be a roll done on a person with an immunity at least once. As in no getting immunity consecutively at all and do away with extensions.