Correct way to measure chain wear?

Started to wonder about the correct way to measure chain wear. Option
1 is to buy a chain checker (e.g. Park CC-2). Option 2 is to measure...

Conventional wisdom is to measure 12 links and if it is greater than
12.125", then the chain is worn and should be replaced. Fine.

However, the chain really has three main places of wear:

1) Inner plate to pin
2) Inner surface of rollers to pin
3) Outer surface of rollers

Now, measuring 12 links only takes into account wear mechanism (1).
It could be that this is the major wear mechanism, as the loads are
most concentrated being the smallest interface area. However, what
the chainrings and cogs care about is the pitch of the chain as measured
from outer surface of one roller to the next. So, if the rollers become
worn so they are "thinner" than they originally were, the pitch of
the chain could increase, even if we the inner plates do not wear.

Question:

1) Are wear mechanisms (2) and (3) negligible?
2) Would it make more sense to measure 12 links from roller surface to
roller surface (using calipers or something of the sort) instead of
pin to pin?
3) If (2) is true, then does the Park CC-2 do the correct thing as it sits
on top of the chain?