1. I suspect that I need to make reservations soon for lunch, but maybe not in October?.
Not been in October, but if during the week, you likely can walk right in, things do get packed on the weekends. It never hurts to make reservations, even if you stop in the day before, or early in the day.
2. Are there other restaurants that also serve cochinillo in Segovia that may not be as popular but just as good.
Certainly, the big places attract a lot of attention (and tourists, and tour groups), but they can get a bit rushed or sloppy in service. We had an excellent meal at Asador David Guijarro. A much smaller place, bit more elegant, excellent food, including cochinillo. They do offer a set menu, and the plates are a bit smaller than what you get at the other restaurants. The ala carte menu is also less broad. We were there several years ago, it appears they are still open, though I had issues trying to bring up their website. As for the other places, they are perfectly fine, each pretty close to the next, very similar menus.
3. When you order a cochinillo, is it enough for 2 people to share or each person orders their own. What side dishes would traditionally compliment this dish.
The pig is very rich, two can get a good taste from a serving, but they would expect that each person order their own entree, so maybe each choose a dish, and share. As for sides, probably the best known is Judiones de la Granja, or stewed large white beans. A potato dish with garlic and onions is also common, as are a number of seasonal vegetables, October? maybe mushrooms.
If not planning to visit interior of Alcazar, good views from cathedral area or worth taxi as close as possible. Currently slight mobility issues but venturing.
You can take a taxi all the way up to the Alcazar, or at least the park in front of it. From there, you have good views on each side.