Great writing, goes without saying with you but I know I should always acknowledge it with and for you, especially before I wax philosophical on the topic at hand.

In my view of the way things work, shaped by my experience and discussions with my not incarnated guides and dearly departed, when we die is one of the things that is predetermined in our soul contracts/ improv scripts/incarnation itineraries, while how we live and how we die can very much be affected by free will, both ours and the dizzying to contemplate intersections with the free will of others. So, no, giving your kidney would only have changed how your mom spent those remaining years, and as you've noted, how your time here would have been spent so far and would be continuing.

As as aside, thoughts of your parents life without you and your sibs are similarly just mental masturbation or masochistic musings All of that is also soul contracted. Free will can affect much but the fact of who our biological family is, and our DNA, that is set in stone. No accidents there.