CIS is a single attack that is very like the sort of attacks that happen in MS. People who get them have a 50% chance of having a second attack and therefore having MS. The risk for people with lesions visible on their MRI is about 80% - so higher, but not 100%.
The reason that people are given a CIS diagnosis instead of MS is because they don’t fully meet the two diagnostic criteria for MS. That means that they have had one attack instead of the two that are required and that they don’t have at least one lesion in at least two different areas typical of MS. I would guess that your wife’s lesions are not in the “right” places to meet that second criterion.
The fact that your wife’s symptoms have worsened suggests that, if it’s MS, there will be more lesions on a new MRI - that’s why the neuro wants to do it. If there are new lesions, then it seems very likely that she’ll be diagnosed with MS, but nothing is ever certain and there are other possibilities.
What else you can do? Tbh, I’m not sure really. Hopefully it won’t be long until the MRI appointment comes through and the neuro can tell you what your wife’s results mean. If things are still inconclusive, then there may be things you can do, e.g. extra tests or a second opinion, but for now, I would think it’s a waiting game: frustrating, but necessary