When you put something in the oven to bake, its very tempting to peak inside. Try to do this as little as possible. When you open the oven door, all the hot air escapes, thus lowering the temperature of your oven. It's OK if that happens a couple times, but if you keep checking...it's going to take FOREVER to finish. Finally, invest in an oven thermometer to know the true temperature of your oven. Some ovens just aren't accurate when reading temperature.
I'm happy you got that passive/aggressive thought off your mind.. Cook on, big fella.
#6 Grocery store chills some vegetables but not others, I do what they do.
#9 Mix sauces similar to mixing paint; alternate top to bottom and outside edge inward.
#12 An accurate meat thermometer is a crucial tool. Learned my lesson after I overcooked a good piece of meat.