What temp does the manifold heat flap open at?

OK, sounds like the flap is working as it should. You just need to warm it up to see if the flap goes more and more open, which would say the t-stat is working. The t-stat won't care if the warm air is from the environment or the heat riser jacket; if the car warms up for a while and draws air from the heat jacket, the flap should open (to snorkel air) while idling due to that alone.