A long time ago I wrote about scouting, and I really think it'd help you to go back and take another look.
The purpose of scouting is not omniscience. Getting that level of scouting would cost too much time and effort and just wouldn't pay off. Instead, scouting is largely to get an idea of what they're doing at the current moment.
You don't need to know every time they build a Starport, or take a new Expansion. You don't have to be on the ball with your army half way there to counter whatever crazy tech switch they're about to pull, if that were the case this game would be all about deception and hiding your next building.
Keep this in mind when you're considering the following.
The number one way people scout is skirmishes. This may seem like a strange concept as it makes sense to want to know what your opponent has before engaging them, but usually the way player figure it out is those quick little back-and-forths in the middle of the map, or the drop in the back of the base, or a fly by with a group of Mutalisks.
The reason these are so telling is they give you an idea of what he has right now, and where he's going. Notice he added on a couple Vikings since your last fly by? Maybe you see a Colossus laser eat your dropping Marines. Personally, I always find Thor missiles to be a good indicator that he has Thors... and probably more coming.
With the exception of maybe some Dark Templar rush or Hidden Void Rays there isn't a lot your opponent can build up in secret that will make a huge difference; and for those sometimes cheesy plays, there is no cheese that can't be handled by standard play. Unless you're going for some crazy Cheese yourself you should be in a good place.
So my advice to you: Stop worrying so much a live a little. Sure maybe when you suicide a Marine for intel he doesn't see a Carrier, but you know what? Unless you're doing something crazy, that's ok. You'll catch them before he has more that 2-3.
Alright Tzenes, if trick plays aren't some unmanagable thing, why am I Scouting?
Scouting is supposed to give you a feel for what your opponent has. Its supposed to give you answer like: BioMech on 3 Bases. Its not for things like: OMG he built a tech lab on his second Starport! Starcraft isn't precise enough a game yet that you can know exactly what that means and how to counter it. Maybe for the first 3-4 minutes of the game, but after that point you just need a general feel.
No Zerg player can go Muta/Infestor/Broodlord/Ultralisk in a undecided game, so chances are if he's producing one of these you probably know what to expect and where he can go next.
*Assuming that you're playing as a protoss.
The expansion is dependent on the map and the opponent you face.
Against zerg on a large map like Shakrus Plateau, the 1 pylon, nexus expand is viable (unless they go 6 pool), because it's almost guaranteed that the zerg will expand as well. It's also because it's a long travel time for them, and a small ramp, they will usually also have a cannon or two to help protect against high aggression for this build because as you put it, they will have a smaller army.
To answer your questions:
1. See above. It is really map dependant, you wouldn't do a 'no-army' expand on a map like Xel'Naga Caverns against terran.
2a) In a PvP it's generally a bad idea to go this build, a 4 gate will crush you like no tomorrow. I normally see this type of build almost exclusively in PvZ. Because of proper scouting they will either respond with their own expansion and you're in for a long term game, or they will apply quick pressure.
2b) Not really sure what the question is, are you asking how to prevent other races from scouting your base as protoss against this build? If so, you really can't, the amount of time required to get a wall to stop a scout is too long. On one base play, normally you chronoboost a stalker to stop the scouting probe/scv/drone, but if you are going for this fast expand, you can't do it. They will see that you are going this build.
3) I don't understand this question.
Best Answer
I assume you are talking about the Photon Overcharge ability that in Heart of the Swarm allowed your Nexus to shoot nearby enemies.
In Legacy of the Void, this ability was changed to be placed on pylons instead of the Nexus.
The ability now reads:
Note also that the range has been shortened in Legacy of the Void to 7.