Multiple endings are very cool! Issues after building are always very annoying >_< It is often hard to find out what the problem is. For me with these kinds of bugs it is usually caused by the execution order of scripts. Eg., if you have two gameobjects that both have a Start method they may run in any order, even though you might want one to run before the other. Perhaps there is a similar issue for you?
Yeah, I see your point on the to-do list. I guess maybe the granny could say to herself something like 'let's do the other tasks first'. Or Maybe you can make the "write a letter to Trevor" appear only after you did the other tasks? I am not sure what the right way to handle it would be and there is probably a better one :/
I am very sorry to hear you lost your grandma recently. This makes me appreciate the game even more. I am very sorry for your loss. I think this was a beautiful little tribute :)