You can open the door, just be quick with it. The oxygen could cause the wood to catch fire. I don’t know how likely that is, but just be quick.
As for the foods. Just keep in mind what is cooking over what so you don’t get cross contamination of raw on to cooked or near cooked food. I don’t know if there are specific thread subjects, but I’ve read where others have done what you are wanting to do. Just plan out when things will be done and time them that way. Nice thing about having a few different racks that you can move up or down.
As for low and high temps, that’s a bit harder but perhaps doable. Cooking hotter just gets things done faster, but?
Depends on what it is that you are smoking I guess.
Hope that helps some. I’m sure others will help as well, give you a few more ideas.