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.
Now I am fairly certain I probably use more than that even, but with the complexity of biological systems it might be as well saying that being shot with 10 bullets is somehow better than with 1000 bullets.
I just point that out, because I am acutely aware how good people are at convincing themselves with statistics of anything, really.
You are actually being hit a lot more by cosmic rays that are not deflected by our atmosphere. We survive this fine. This is probably a more acurate simile than the bullet one, based on the amounts involved.
