@trazzy We did not know before we saw a neurologist that problems with gait and balance while walking can be a symptom of mild cognitive impairment (for my wife's father.) He was 78. Physical therapy helped, but the physical therapist we saw stopped treatment because he was not continuing to progress. In his case, the therapy was helping to prevent further decline which that therapist did not think met insurance guidelines. I would get another therapist now in that situation.
I mention this because the other symptoms may also be neurological. Foggy, forgetful, and "not hearing" is a frequently used proxy for not processing the same as before. The brain scan recommendation goes with this line of thought as does the geriatrician. In our context we do not have many geriatricians, and neurologists are usually the ones dealing with these concerns, so that may be an alternative referral possibility.
Of course, it could also be spread of the cancer although the oncologist is apparently ruling that out at this time.
Regardless, our hearts go out to you both with this multifaceted set of challenges.