Not sure if I can answer all your questions, but you will most likely have a lead time on your rate change.(ex: 45 days before each change date). That means that if your rate is scheduled to change on the payment due date of 8/1, you would be using the index as of 45 days prior to that day. Once the rate changes, the payment would change for the next billing cycle and the new payment would then be affective 9/1.
"It's time for the Jedi to end."