1. OK, 16" o.c. is standard.
2. A span of 11'9" is too much for tile. You need to either sister or install a supporting bean under across those joists. Make sure they all touch, maybe steel I-beam? I think I'd rather sister if there's any obstructions at all.
3. Well OK, you're gonna sister or put in a beam so doesn't matter.
4. Make sure you do NOT buy pressure treated "outdoor" plywood. Get 3/4" subfloor grade plywood, it'll say something like "exposure 1". The underlayment can be thinner, like 3/8" or 1/2" or thicker if you need the height. The more the better. Ditra is a good idea, better than cement board and only adds 1/8", so you can go with thicker plywood.
5. Sounds good cuz you've already done #2.