Question: If you depart from a 7400 ft runway at max Landing weight and loose an engine in a 737-800 can you return to land at that field.
Yes you can. In a perfect situation you will have plenty of room assuming max manual braking (about 1250m to stop from memory) but I think you already know that.
so flaps 30 is still more than adequate, and 40 being the best choice.
Depending on the braking scheduled IMO.
The s/e example is an emergency situation and therefore no factoring is required (as I am sure you know) .
Basically, by choosing Flap 30 (and I'm not saying this crew did), unless you select autobrake max your buffer for speed maintenance, threshold crossing height, touchdown point, float in the flare, tailwind etc is only 370m. Not enough in my opinion when simply selecting Flap 40 brings you an extra 200m of buffer. Of course it can be done at flap 30, but why would you?
If you applied max braking you could stop in 1000m....but why would you?
stack the deck in your favour and make sound choices, Flap 30 is not a sound choice on a runway 2200m long unless gusts are a factor IMO.