Yes. You only get paid once you reach $100. If you make less than that, Adsense rolls it over to the following month, and so on.
It's set there, you may not change the minimum required amount to be earned. They do that so they don't waste their time and efforts on people earning less. It would be worthless to send out checks for $5.
Not at present. Google checkout is aimed ad Merchants who actually sell a product and isn't strictly a person to person payment method like Paypal. However I won't be suprised if Google uses Google Checkout as a stepping block to create a system that could rival Paypal. Imagine receiving (micro)payments from other people, paying Adwords with Adsense earnings etc. When it comes to cashing out they just add it to your Adsense cheques. Hopefully it's just a matter of time. (I hope! )