RE: Confused about Resizing and Head Space
Looks like bigcountry answered your questions pretty well. but to extend the answer on case life: It can vary by manufacturer and even by lot for a given manufacturer. Your powder charge can also affect case life. Every time you fire a round, the brass 'flows' toward the case mouth - hence the need for trimming.
The area most affected by this flowing of the brass is the area around the web. Besides neck splits and loose primer pockets you will need to examine this area and watch for a bright ring around the case. If the brass gets too thin here, your case will rupture or even completely come apart.
Also, understanding how a FL die works, if you back the die out of the press (not adjusted to the manf. instructions) it will only size a portion of the body and neck and will not touch the shoulder. The amount of sizing of these areas depends on how far you back it out.