Yes, your diagnosis is correct. If the oil filter is not tightened to spec, or if the "O" rings are not positioned properly, oil will leak, which is what caused your serpentine belt to come off.

If you are going to put the same serpentine belt back on, be sure to get all of the oil off of it first, and hope that the oil has not penetrated the belt. A belt flying off can cause some serious damage. You may want to get a new one.

As far as the serpentine belt diagram, do you have an SRT or NA?

This is the diagram for the SRT: