Examining your questions in order:
The general rule is onions first. Sauté the garlic towards the end for 30-ish seconds before removing from the heat.
As ElendilTheTall correctly pointed out, garlic can scorch quickly, especially if you tend to sauté on the hot side (as I do). Starting your sauté with onions first has two advantages: it allows you to better control the time the garlic spends on the heat and the moisture released by the onions on the heat provides a buffer, of sorts, from the otherwise dry heat of the sauté pan.
Both my training and my experience lead me to believe there is no conceivable advantage to placing garlic in a sauté before onion.
The sauté time to take that bite out of raw garlic certainly depends on the way the garlic is prepared prior to the sauté - the 30-ish second recommendation I gave in (1) above would be for minced garlic. If you tend to use chunks or slices of garlic, you'll probably need more sauté time to mellow the garlic out - in that case, you should still go with the onions first, but you might need to sauté longer over lower heat to make sure the garlic has a chance to mellow throughout before the exterior burns.
Finally, as a personal note, the only time I ever include my garlic with the onions at the same time is when I am sautéing bell peppers with the onions. Because of all the moisture that is released between the peppers and the onions, I find that it takes a lot longer for the garlic to cook sufficiently so I let it sauté with its buddies for the full several minutes it takes to sweat them.