1./ Your door gusset idea won't work. When you try to close the door it will hit the grates.
2./ I don't weld braces on anything. The steel rod is just a 3/4" bar I hammer into place and the whole tube conforms. I then weld in the guides and it doesn't move after that. I remove the rod after I have welded in the guides.
3./ Use a 9" grinder with a 1/8" cutting disc to cut the doors. (Watch out they are nasty S.O.B's) but get the job done fast and clean.
4./ Make sure the weld seam is at 6 o'clock and take all your measurements from there. ( wrap a tape around the outside and divide in 1/2 ,1/4 to get your 12 o'clock and 3 o'clock).
5./ On a 20" tube I make the door cut at 75mm (3") past 12 and 3" below the 3 o'clock for maximum clearance and can get 2 grates in there , no problem.
6./Cut a compressor tank that is 40 years old and they will spring. cooking on it won't change anything.
7./ If you get your end plates plasma cut they will be perfectly round. Cut them to halfway between the ID and OD to give you a nice groove to weld. Stand the tube on it's end when doing this. MAKE SURE you have welded on your tabs to stop the grates tipping as you can't get the welder in there after the end plates are on.
8./ I would mount the door BEFORE welding the end plates on so if it is out of shape you can make the tube conform.