I don't know anything about the borrowing/banking question, but I know a lot about the challenges of caring for a loved one with Alzheimer's, and the impact it can have on the primary and secondary caregivers. My mother was diagnosed with Alzheimer's at only 49 years old, and my father became her primary caregiver, eventually selling his company and retiring young to have more time with her. Fortunately he was financially able to do so, as it's an expensive disease that can bankrupt many people (her care at the end surpassed $25,000 per month)!!!
Eventually, we (the five children) staged an intervention and asked him to put my mom into an assisted care facility. The stress on him was going to kill him, too, and we didn't want to be orphans (granted, all adults, but too young to lose both parents).
After doing so, he suddenly had far too much time on his hands, despite visiting mom every day in the assisted living home, and became very involved with the Alzheimer's Association. One of the things he learned about is the availability of respite care for families who choose to (or, in some cases, must) keep their loved ones at home. It is generally subsidized based on the family's needs and the caregivers are trained and professional.
My point, after the rambling back story, is that I understand your desire and need to be with your MIL, but for your own health and life, it's important that you make some time for yourselves on occasion. I know that tremendous guilt can precede or follow these decisions, but it's truly for the best and not at all selfish!
Just my two cents, not knowing the specifics of your situation. It is a devastating disease and you have my empathy during this difficult time!