Every airplane maintenance manual has some limitation for how many fasteners can be missing, generally predicated on Leading/trailing edge, how many in sequence, how close to a corner, total number or %-age.

There are also a bunch of drain holes, etc on an airframe that look like a fastener is missing, but it's not.

On my walk-arounds, I'm generally not looking for things that can't be seen from the ground. Because I'm on the ground and by definition I can't see it. From the other thread there were apparently 32 million flights where the walkaround inspection was good enough (Southwest excluded) that the flight went fine. I'm unaware of any issues that brought the plane down that would have been caught by getting a look at the parts I can't see from the ground. Generally the only time I go back and take a look at the wing from the cabin is if I'm concerned about Ice on the upper surface of the wing and deciding if we need to be de-iced or not.

The wings of modern airliners are typically made up of multiple different materials. Aluminum, fiberglass, carbon fiber, etc. You can't tell if the "epoxy" is cracking or delaminating from looking at those pics. If in doubt or during major inspections they will bring out the special equipment to check.

Each airliner has a Configuration Deviation List that allows you to fly revenue flights with parts missing. I flew one two weeks ago with the entire door missing from the lavatory service panel where they dump the blue juice. Fairings and non-structural panels are missing all the time. There is typically an adjustment required for fuel burn due to increased drag or a limitation on how many flights or flying days you can go before it has to be replaced.

Each of these airplanes gets a good maintenance inspection every 100 hrs, so every 4 or so days for major airlines. A pilot is doing a walk-around between each flight to check for major issues (unless you're flying Southwest). If you want to freak out about every dent, paint scrape, sealant crack, drip, you can find a reason not to fly.