1. Startups have happiness potential for developer not only because of the monetary wealth but also because of the
wealth of the challenges faced. Developers live for tricky puzzles to solve and startups are one place that you can get them. Startups aren't the only place where you can find challenges (as you've noted).
The monetary wealth typically comes from stock options. Stock options are a promise that you can buy stocks at a certain price at some point in the future (when they become available, e.g. during an investment or IPO). Options are how you become an overnight millionaire. Startups aren't the only place where you can earn options.
You'll typically find that companies that aren't publicly traded have stock options (ask about them in your interview). It's not something that's exclusive to startups - I have options in a 10 year old company and have cashed in some of those options.
2. Yes.
3. Accrue wealth like anyone would. Developers earn relatively high salaries (whether they work for a startup or not) and hence have an easier time getting into the situation where money works for them.
However, even one of those developers who earns $1 000 000 can have no wealth if they waste it all. Someone who earns $75 000 can amass a fortune. It all comes down to how well you manage your money. If you do something to get rich quick chances are you are going to end up penniless. It takes time, discipline and a brain.
There is no quick road to material wealth.
Money does not make you happy, it merely multiplies what is already there. It's a catalyst. If you're already happy, money makes you happier. If you're already sad, money will make you miserable.