[Rodger Knox]

If your not building the guitar, just forget about the straightedge test.

If the relief (adjusted by the truss rod) is OK (0.003" to 0.008"), the action is to your liking, the string height above the top just in front of the bridge is OK (7/16" to 9/16"), and there's adequate saddle showing above the bridge (more than 1/16"), then the neck angle is fine.

In other words, if the guitar can be set up to play without sanding down the saddle beyond the above guidelines, the neck is OK.

The straightedge test is done without string tension, and the neck adjusted to be dead flat, so that the straightedge touches every fret up to the 12th or 14th, beyond that there can be small gaps (called fallaway).

The straightedge test is useful when initially setting the neck, because you can't put strings on until the neck is on, and when resetting the neck to evaluate how much change is needed initially and to monitor the progress as the work is done. That's really the only time it's useful.

If your not building the guitar, or resetting the neck yourself, just forget about the straightedge test.

edit: I forgot to answer your questions
1.That's not exactly true, but probably close enough. Some guitars may need a reset in just a few years, others never will. Wood varies.
2. Yes, but that's hard on strings, so probably not worth the effort unless it will be a month or two before it's played again.
3. Yes, and so will lighter strings, but it may not sound as good.
4. The $350 probably covers finish repairs.
5. You CAN be certain the neck is OK without the straightedge test.
6. No, that's one reason the neck needs to be adjusted dead flat to use the straightedge test.