I'm very sorry to hear about your mother's condition or rather what we call "de-conditioning" since her recent major surgeries. It's difficult to accurately comment on an MRI that I have never seen (we never trust anyone's report unless we personally review them). But just going by the report you provide, it does NOT explain her current state. Small vessel disease is a term we use to describe little "ditzels" throughout the brain of no clinical significance in someone who has many stroke and heart risk factors such as your mom. Remote is a key word here. There appears to be evidence that your mom had small strokes on the right side of her brain and brainstem a while ago. If she did not have an episode or bout of left sided weakness in the past, then it is likely that the infarcts (strokes) described were clinically silent.
It is quite possible that she is requiring a lot of time to recover from her surgeries. This is something we see commonly at the CLeveland Clinic as we have many heart surgeries done here. Older patients coming out of very major, emergent heart surgeries as your mother can have a rough post op course and need more time and physical therapy to recover than a younger patient who went in for elective heart surgery. Her mental outlook certainly has an impact on her recovery as your docs have mentioned. If that MRI is recent and the reading is correct, there doesn't appear to be a neurological reason for her not to be able to walk from a brain standpoint. On the other hand, her medical/cardiac condition needs to be evaluated and management optimized for adequate recovery. Best of luck to you and your mother.