Crimson offers a great reply and gets a thumbs up from me. I'll just add a bit.
Whether or not you submit a sitemap, Google will visit your site as long as it knows the site exists. If your site offers solid navigation, there is absolutely no need to submit a sitemap. Google will find and crawl all of your pages. If you have coding issues on your site, navigation issues, island pages, etc. then a sitemap is helpful so Google can be aware of these pages it would otherwise not be able to find.
With the above noted, a sitemap is easy to set up and automate. You can pretty much "set it and forget it" so it's still a good practice. About your questions,
1. It's your call. If a page is linked to in your main navigation such as About or FAQ then Google should find it 100% of the time. There is no need to include it in your sitemap but there is no harm either. Either way works.
2. Yes, as per the above as long as Google can find the page it will index them. You can even have horrible coding and navigation and Google may locate your pages if you have earned external links to them from credible sources.
3. Last I checked a sitemap can hold 50k URLs. If your site has more then 50k URLs, then you can break up the sitemaps into smaller files. The advice Crimson shared is correct.
In summary, if you implement all best practices in your site design and do not have any island pages then a sitemap is not needed but it is a nice backup.