[1a] Words in a word-search are visible and hidden.
[1b] If seeing something makes something not hidden, then camouflage would be pointless. I would like to think that this notion is not intended.
[1c] Harry Houdini was killed by a punch to the gut because he could not prepare himself for the punch. I am sure he saw the man, and the attack to punch him.
[1d] It is not only seeing the danger that prepares oneself to react. You also need time to react, and perhaps some training on how to react.
[1e] Someone in a monster mask popping up and going boo has more of a chance to surprise you than someone walking up to you in a monster mask and going boo.
[2a] Observing and seeing would seem to be two different things.
[2b] Observe: transitive verb to see or notice something, especially while watching carefully
[2c] See: transitive and intransitive verb to perceive, or perceive something, with the eyes
[2d] One cannot watch something carefully if one does not know it is there. This is what investigation/perception is for. Investigation/perception helps you notice things that you see.
[2e] Your boss seeing you is not the same thing as your boss observing you, for example.
[2f] Light does not stop working, you just stop noticing the light your eyes see. Noticing the important parts of the light is investigation/perception.
[2g] I would likely play it something like this: immediately after the attack there would be a Stealth versus investigation/perception roll (with the active person's DC being the passive persons result, the hider gets tie breakers), and [ii] the defender would be able to actively look for the attacker on the defenders turn, and then have a Stealth versus investigation/perception roll (with the active person's DC being the passive persons result, the seeker gets tie breakers). I would allow the seeker to either make a spot check, and then attack (normal if successful, and limited if unsuccessful . . . no attack at all and/or an attack at a disadvantage . . . perhaps depending on how much he missed the check by), or I would give the seeker advantage to see the attacker if he readied an action to spot the next attack and not attack. In any event the seeker should go for cover.
[3a] A young man is looking around at the beach. He can see the waves, the sand, the rocks, the sky, the seagulls, the boats, the dogs catching frizz-bees, the kites, the towels, the coolers, the kids, the men, and the women. He is observing the young women at the beach via actively looking at them and noticing them with intention.
[3b] If a crab darted out of hiding and pinched him, then the crab would gain advantage (if the young man failed his investigation/perception check).
[3c] If another young man walked up to him and punched him for looking at his girlfriend, then the attacker could have advantage if the attacker wanted (if the defending young man failed his investigation/perception check).
[3d] If a group of young ladies walked passed him, from behind, and one of them slapped him on the behind, then he would be at a disadvantage to figure out what young lady slapped him.