1. Yes, it is working with embedding videos. You need up to $100 to start, no need to SEO video pages. You need to do SEO on homepage, and / or on some video pages but only after keyword researching and optimizing these pages for high search volumes and low competition keywords. The same you can do for tags, categories, etc. Choice is your. It is better to launch site with full content added, or later add not to much pages per day - as example 10 - if you go more, it is a risk for 4 index results penalty.
2. I can't tell exact number, but if you put ads too early - it is a risk to get 4 results index penalty too. Most people say it is enough to have some organic traffic.
3. If you submit sitemaps, you can get more pages indexed in shorter time.
More info about 4 index results penalty:
https://www.blackhatworld.com/seo/an...y-now.1318746/
This is actual for new sites.
If you want to make money from tubes - you will need huge amounts of traffic.
To earn from cam sites you'll need way less traffic.