Looks like default in BeerSmith is Tinseth. In my experience, IBU's in Tinseth are generally lower than others (Jamil Zainasheff uses Rager for ex), especially for late hop additions.
Rager gives you 39.7 IBU
Tinseth: 28.6
I'd say thats definitely acceptable for an APA
How would you categorize New Belgium 1554? To me probably most similar to a dark american brown ale or a robust porter.
TastyBrew.com calculator lists those styles with average volumes:
American Brown - 1.5 - 2.5
Robust Porter - 1.8 - 2.5
According to TastyBrew.com's priming calculator, 1.75 Volumes CO2 would require 2oz Corn Sugar for 4 Gal @ 70F. So I think your measurements are correct.
How does the presence of yeast on the bottom of the vessel discourage more yeast from dropping out?
This might sound like a smartass question, but I assure you, I am legitimately asking.
Not sure if its the curse, but my second batch "finished" out at 1022, only to explode 4 times in my closet. You can probably bottle it and watch them like a hawk. If they get a bit over-carbonated then just throw them in the fridge
Do whatever you like. My first 3 AG batches were 3-3.5 gallons, because like you, I didn't have a large enough pot. The problem was that I didn't create enough beer. Now I'm back to 5 gal batches :)