Regarding your thermostat, I would wait and see whether it actually does climb when the weather gets warmer. From other posts on this board, after putting in a new thermostat, some people have higher operating temperatures. So if a brand new t-stat tends to make the reading slightly above the 3-o-clock reading, then I wouldn't worry about it that much; as it sounds like your t-stat is behaving like a brand new t-stat. Just pay close attention to it as the weather warms up.

Regarding the sqealing, try to localize it. Many things could be causing this. A few are alternator bearings, water pumps, pulleys, the belt itself. Try to get one of those stethoscopes (sp?) with a metal rod. You should be able to get this at an auto parts store. With the engine running, touch the rod to different components to try to localize the sound. I'm sure you don't want to replace anything you don't have to.

I wouldn't worry about the central air duct in the front. Only the two side ducts came with plastic inserts. You can tell because the two side inlets have four tabs that allow the insert to clip into place, while the central duct does not.