I'm glad to hear your first experience with your foundry was a good one. I only melt aluminum as a hobby but have used Dave Gingery's books to build the metal lathe, metal shaper, and horizontal miller. From my experience melting aluminum scrap in my homemade foundry:
1) I don't think degassing will be necessary for the ingots.
2) The dross will accumulate at the top of the melt but even after removing the bulk of it there will be a thin skin of it over the surface of the metal. I would not be concerned with that as when you pour the metal it normally runs out of the crucible/pot from under that skin and it has not caused me any perceptible problems with my castings. There always is a small amount of dross and aluminum that clings to the pot and the molten metal that pours usually is good in my experience.
3) you may find more definite information regarding the flow around the crucible but I would think that as long as there is a good air flow you will be OK, and probably the less air flow will allow you to retain more heat in the furnace.
What are you using to fire your furnace? I have only used charcoal so far with very good results, but a larger furnace with perhaps a propane heat source would be more functional.
Thanks for posting your thread and keep the information coming along with some pictures when you get time.