Three good questions
1. How long can you let the scratch coat sit before covering with the brown coat? Is there a problem in letting the scratch coat sit for 30 or 60 days before covering with the brown coat?
We usually let the scratch coat sit for 2 days on metal lath, overnight is fine in hot weather. If not your brown coat can sag on you. I let the brown coat set a minimum of 3 days, usually a week before putting on the finish.
You can let the scratch coat as long as you want. After a long time you may want to soak the wall real good with a hose the day before you brown, and right before.
2. When you install rib lath on a soffit, do you install the ribs against the rafters (ribs up), or with the ribs down. It seems to me the ribs should be facing down. This results in the ribs providing a natural screed, and allows the lath to sit flat against the rafters.
The most important thing is the open end of the V should go up-If not you will have a crack every 4 inches. I didn't say may crack, I said it definitely will. Diamond mesh lath works better for stucco.
3. Do you have any techniques for stuccoing a radius wall ?
A cool trick for rodding radius walls is to nail ropes or old lengths of hose crossways around your curved wall 5 feet or so apart. We use a 5/8" rope nailed on the lath.
Using the ropes for screeds, rod off side to side between the screeds. Make sure you take the ropes out. In the old days, they would use old hoses for barrel ceilings in plaster. You can leave the hoses in plaster, not stucco, and not have cracks. I learned all this from old plasterers.