Ok after selling camping gear for about ten years;

Yes I guess in Canada and Europe they would use celcius.

Tents do make a difference in the actual camping scenario but not in the rating.
The way REI CONFIRMS manufacturer ratings is to use a "dummy" filled with 98.6 degree F water in a bag to be tested. Then they put it in a special "freezer". The dummy has thermal sensors; as soon as the water drops to 98.5 the rating is made. So if the water drops at around Zero they will rate it Zero.
Manufacturers do not employ a tent when they rate a bag. In fact many simply use a formula that takes into account the thickness of the insulation, that is all.

Manufacturers do not have a survival rating for their bags. see above. I have personally spent a night out or two in temperatures that were below the bags rating. Including an ice climbing trip when it got down to 12 and my old worn out TNF Cat's Meow was rated at 20 (when it was new; synthetics compress and lose insulation over time). If the rating was a survival rating then would'nt I be dead?

Yes each manufacturer may use their own method but these are always a ball park and there are some makers that are notoriously consevative and others who are not.

Yes you need to wear appropriate clothes like synthetics etc. and just like having too warm a bag can make you sweat so can overdressing.
Many climbers, mountaineers and ultralight backpackers will deliberately use a bag that is rated above the expected temps in the plan to use every ounce of their clothes at night. Two books that illustrate this are; Beyond Backpacking by Ray Jardine and Extreme Mountaineering by Marc Twight. Jardine is the father of UL backpacking and Twight is one of the best mountaineers in the world today. I would say that they are experts.

I guess I could call my buddy at Mountain Hardware tomorrow; he is their one of their designers. He should confirm what I say.