you are right about the 19mm having to be loosened to adjust the belt tension.
first a suggestion for all, weld a bolt(i used a 17mm head) to a short 19mm socket and that will give you a way to get at the 19mm with a 17mm wrench.(no damage to anything while using)
if i had your situation i would adjust the belt as tight as i could get,then pull or push on the belt to see if it repeated its position,if the rubber is broke from the center sleeve then the belt should get loose as you pull on the belt,else it stays tight and isnt slipping so tensioner should be good.
hope this helps.
larry perkins lou ky