When sauteing should I put onion or garlic first?

Onions always benefit from a few minutes on their own to soften and start sweetening. Garlic burns easily, especially when finely chopped or crushed, so in general should not be fried as long as onion. Having said that, when doing a quick stir fry or similar dish, you can throw in the garlic first for 10-20 seconds so that it flavours the oil.

Examining your questions in order:

1. The general rule is onions first. Sauté the garlic towards the end for 30-ish seconds before removing from the heat.

2. 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.

3. 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.

My answer would be "after the onions".

I had a chef tell me that garlic (and black pepper) burn around 140° C (284° F). You can guess this is quite low if you've burned garlic as often as I have before.

I'd suggest either controlling the heat, or as Stephen Eure mentionned: cooking along something moisty to avoid direct high heat.