Wow, there's a lot here. A few thoughts from another domestic traveler (typically 85-95 segments per year) to get you started. I'm assuming UA Is your choice.
1. For domestic travel, you have to buy a membership. For me, it's been worth it over the last few years, but others will say not worth the money. For international, you need to have gold with UA (or buy the membership) which is Star Alliance Gold and gets you lounge access. There is also a club access credit card.
2. Check out the Star Alliance website for a full list of partners
3. Sometimes. Call UA, tell them you're starting to travel a lot, and ask for a status challenge. IMO, your goal should be to get to Gold status ASAP.
4. Too much to list here, suggest you check out the benefits on the UA website.
5. "Regularly" won't happen for most people. Most people report little success with free upgrades until they get to 1K status (100,000 qualifying miles). And then it depends on your routes and other factors.
6. To some extent this is true when you compare to 5-10 years ago. But when you travel as often as you will, lots of stuff goes wrong. Having status makes you much less miserable than those without status, and I think you'll appreciate it most then.