I think that it is a foregone conclusion who they will pick.
In fairness to Monty he has bowled really well, and it is encouraging that England have a quality back-up. It would be strange if Swann pushed himself too much in the warm-up. He is potentially not 100% and no spinner worth their salt would show their full hand before the big stuff - see Warne in England in 1993.
On the other hand, Pakistan are notoriously suspect against quality left-arm spin, so Monty playing may not be the most surprising thing - if only England still had Colly's wobblers...