I thought that while the spare tiles did exist, there was never an actual safe procedure for replacing tiles (that didn't require being docked to the ISS) they were only carried to be available when the choice was between losing the entire crew on reentry or risking a crew member?

I don't quite understand how the airplane shape made it easier to model the loading and positioning? (Not saying you're wrong, just doesn't fit my intuition and I'm curious).

My understanding is that Shuttle didn't have to answer the questions about tile gaps etc because it used glue rather than mechanical attachments, if that's what you mean by positioning.