The lethal dose is 5-10 grams, the recommended safe dose is 10 milligrams maximum. Between those points you'll experience some pain and stomach discomfort. Regularly going over the safe dose may cause some organ issues.

Basically, even if you swallow all your toothpaste you're unlikely to have problems unless your diet is already rich in fluoride, and spitting but not rinsing is fine.

That's a good question. Having not seen reliable research on that point, my approach is as follows: I use a small amount of toothpaste (say, 1/4th of what you see in ads or less) and I spit excess. In this way in the worst scenario I digest at least one order of magnitude less fluoride than the threshold of the safe amount.