> What I'm interested in knowing when you get to it is how these two came to collaborate -- was Powell originally helming it with his team and Buckley came in to help in the midst of his bereavement, was it the other way around, Powell coming to Buckley's aid, or was it always a collaboration? (The orchestration/add'l music credits indicates this may have been Powell's joint, but that's my speculation.)

It's a mystery. There is one track solely credited to Powell. Everything else blends in Buckley and Powell's team. There was likely no 'collaboration'.

Going off of how Buckley described things here - https://www.youtube.com/watch?v=ES45J9IHeaU - it seem this was at some point 'his' project and he was told to ape the trilogy's style.

Did Powell start and realize he really WAS sick of action movies? Did his personal tragedy play a part? Did Buckley get replaced by Powell's team after the filmmakers got nervous? All possible. Little public confirmation of anything.