1) Yes, but as SEP said, a shelter is recommendable.
2) Probably yes, in such a large enclosure, but in general it's not recommended to have multiple males in the same enclosure. I'd probably cut it down to 3-4 males. And yes, you can eat fertilized eggs.
3) Yes for the table scraps, but as SEP pointed out, the protein in chicken layer feed is usually quite low. I believe research shows that coturnix quail need around 24% protein. Once grown, they can survive on less, but the egg production is likely to suffer. People usually use game bird or turkey starter.
4) Avocado is probably the most likely mistake - it is poisonous to birds. Other than that, just use common sense and don't feed weird stuff like coffee, chocolate, overly sugary or salty foods, raw meat or things like house plants that might be poisonous.
5) At least 1 square foot per quail (assuming they are coturnix), but larger is always better. If the height is above 12 inches and below ~5 feet, there is a risk of boinking injuries as SEP mentioned. You can pad the roof with foam rubber, make it from soft netting or similar to prevent this. Coturnix quail can handle cold weather, but they should be able to get out of the wind and the rain. Some people say their birds won't use ramps to get into a shelter, others say their birds use the ramp just fine as long as it's not too steep.
6) On the grit thing SEP mentioned - that is actually not required when you feed them pellets. Those are water soluble. But if you feed them seeds, they should be supplied with grit. And as SEP mentioned, they need extra calcium when laying.
7) In general it is not recommended to introduce new birds into a flock - they do best when raised together. But it works for some, and with the size of your enclosure, you might be one of those. There are several tricks to reduce territorial behavior of the birds and make it more likely that the introduction will be smooth. Those include moving the original birds to a different enclosure and keeping the two flocks right beside each other (separated by wire so they can see each other) for 1-2 weeks before putting them all together in the original enclosure. This way the enclosure will be 'new' to all and the birds will be used to the presence of each other before being placed together.
You can inbreed for a few generations, but fertility of the eggs will start to decline if you do it for too long.