What if it's not dementia, alzheimers or a stroke? Earlier in the season, Rebecca mentioned that she basically takes Advil like it's candy. What if the car accident caused her chronic pain that got so bad in her old age, that she was confined to a wheelchair, etc, and was opting for assisted suicide? It would explain the calmness of everyone (this had been discussed and planned) and the games - it was her "going away" party. It's why Tess was reluctant to see her, because she knew she'd have to watch her die. She was also probably super drugged up, like she was in the hospital after her accident. Which is why Randall introduced himself as her son.
Just thinkin' outside the box...
Maybe Kevin decided to adopt and just happened to adopt a kid with similar features? He would absolutely have Kevin's personality - nature vs. nurture and all that. I was adopted and people are always surprised because I look so much like my family, and my Dad and I both sing. It would be a nice, full circle thing, too.
UGH, I'm so tired of shows ending so early now! I remember when I was a kid, the seasons would go at least through May. Show seasons ending always seemed to coincide with the school year ending.