Spoil priority only works when taking items from inventory (for example chests), it has no effects for belts as stated by the tooltip for spoiled-priority. As for taking only last item on line from a belt, that is the same as 119003 for which i am considering current topic to be a duplicate of.
I encountered this, too. I assumed that it applied to belts. Why wouldn't it? That seems intuitive to me. (I can force this behavior in game by using chests and circuits, but it seems like that should work from belts.)
PS: I was about to post a bug about this not working on belts, but I see that it's intended. Can that please be an option?