Fairly certain its the right belt. Continental 6PK875 for my 05 Mini Cooper no AC (old one was an 870 even). All pulleys have a pretty big lip, the crank mmaybe the smallest. The tensioner is pinned away.
From that position a gentle rotation of the engine using the pulley bolt will guide it on
Make the tensioner the last pulley you put the belt onto, no ridge(s) to battle with. Also possible you have the wrong belt (even though you state you have an 875 even with the old was an 870). If it just won’t go with the tensioner fully slacked then it’s routed incorrectly or the wrong belt
If you pull the tensioner, and put a pin in it to hold it in place. Then it with fly over the pulley. Then pull tensioner again remove pin and belt should be in place.
Manged to do it, one person held the tensioner as far back as possible past the pin, rhe other levered the belt on the crank, really tight but did it