Matt,
The 5 questions you have asked in your post are the same 5 questions that most techs ask themselves every day.
There are no easy answers. If you ask 100 techs you will get 100 different opinions.
The best I can do is simply offer my own opinions to you.

1) I prefer independent garages. I like working on different types of vehicles, rather than one line. Tooling is sometimes an issue at indy garages but there is never a dull day.
2)I don't work on flat rate. I get paid hourly and prefer it that way. This is another reason I don't want to work in a dealer, because practically all dealers are flat rate. When I did work flat rate, it was GREAT on weeks where there were plenty of cars and everything went smooth. On weeks where there were problem cars or slow days, it was terrible. That's the nature of life on flat rate. Whether or not flat rate is good also depends on what kind of jobs your shop is giving you and how good the service writer is at making sure to get all the labor hours that he should for a job. You can make good money on flat rate EASILY if you are doing brake jobs, ball joints, and water pumps all the time. If you get stuck with all the diagnostics, it's a lot tougher to turn out big hours, especially if the shop doesn't charge appropriately for diagnostics.
3) There are lots of guys providing for their families in this business. Some better than others. Where you are going to be living/working will have a big impact on what kind of life you can provide for your family. Rural areas are cheaper to live in, but pay rates are also lower. Cities have better pay usually, but higher cost of living as well. You will have to try and find a balance that works for you.
4) Location, location, location ... How techs work and survive in one state/city is going to be very different from how techs work and survive in another. Once you have some ideas on locations, try to talk to techs in that area and find out what sort of pay/benefits they are getting, what their rent/mortgage costs, local taxes, etc. I can't stress enough how much things vary from one place to another. In some places, techs make 6 figures easily. In others, the best of techs struggle to make 35k a year.
5) That's up to you. I can say that there are other careers that will pay as well or better with less stress and physical wear and tear. Many of them may require a degree. Someone who is mechanically inclined and has a 2 year degree from a community college has any number of options. For example, I hear that many factories have repair techs for equipment that get paid quite well.
But you have to ask yourself if you would be happy without the daily pressure and challenge of the modern automobile ...

Personally, although I often talk/dream of leaving this industry ... I just don't know if I would feel satisfied without the constant technical challenges of complex modern cars staring me in the face every day