So I made some changes to the 1-click. I got rid of my clocks and set them up to spawn as a part of the 1-click command. But the result is the same. I end up with a tower of minecarts. Hrmm..

Oh man.. Ok. Figured it out. I run liteloader in my client for some client-side mods. Apparently something in there is interfering with me running 1-click commands. Turned off my mods and ran the 1-click again and it works fine! Yay!

ToDo:

1) Cleanup silverfish near passive mobs to prevent any possible issues

2) Prevent mobs that spawn outside the protected area from despawning if they enter the area. That way you can't escape a mob just by running back inside.

I have a recommendation to "stop spawning." You can make it so that any mobs just outside of the radius get a nametag, and you can change the module to only send the mobs into the void if they dont have one, that way any mobs who want can wander in without being killed instantly.

I didn't want to make it too much more complicated.  If passive mobs does not cause too much of a problem, then we don't need to worry about the extra command blocks.

Hmmm. rather than removing silverfish from minecart rails, could minecarts remove the invisible silverfish in its area?  But then there are multiple types of minecarts, including spawner minecarts.  Looking... 8 different IDs... arrrgghhh...

Really the effects need testing, rather than just preventing.  You were saying the reason for the silverfish is to stop mobs from 'constantally flashing' around the player all the time.  Is this really such a problem?

In my earliest versions, it just teleported everything out of existence constantly. I found it really distracting when playing with it like this, especially at night. You'd see packs of mobs spawn, then slide through the ground as they get teleported down (really strange visual bug with the teleport). Since there are effectively no mobs in the area, the game is constantly loading in loads of mobs every second. I'd describe it as raining mobs.

With the silverfish solution, after you've been in the area for a few seconds, new mob spawns are effectively disabled. The game sees there are enough mobs around and doesn't try to add more, except to replace the few that despawn randomly at the edges.

As for the rails, it was pretty simple to do, and actually only took 4 commands, one for each rail type. If you ignore the data value, it checks for all possible directions for that type in one command.

So I've run into a bit of a snag with the Nether....

I've been play testing for the last week and I just recently had cause to go into the nether to get to my Mesa biome for clay. When I got out, it was full night and not a monster in sight. No scarecrows are setup out there, and no torches. This was actually my first visit to the biome.

So I did a quick /kill @e[name=HIDDEN] to see, and it killed a whole pile of mobs :( Seconds later spawns started happening normally.

But prior to this, I had tested repeatedly by simply walking away from the scarecrow protecting my base. Once I got outside of its range, I quickly began seeing mobs spawn as normal. 

So there is something special going on when I teleport out of the area via the nether (and probably via the TP command, still to test) that is not happening when I simply walk away on foot. I have no idea what. If anyone has any idea why the mobs would not despawn normally when I go through the nether, I'd be greatly appreciative. Otherwise I think I'm going to have to build in some sort of kill switch that gets rid of all the placeholder mobs as soon as the scarecrow detects that there are no players in the area. 

Edit: Tested again and now even when I walk away nothing is spawning. Hrm... So the server is not cleaning up the mobs properly from the unloaded chunks.

So silly, did some more reading and more testing. Being too hasty. 

Takes time for the mobs that are out of range to despawn. So if I go into the nether, it unloads the chunks and does not process them further till I get back to the overworld. At which point it sees I'm more than 128 blocks from the mobs and starts to despawn them. I got a slow trickle of new spawns this time as I sat around outside my mesa portal.

So no problem, just me not thinking too hard.

Of course that would only happen if the scarecrow hidden mobs were in spawn chunks!

If they are not then the chunks unloaded by the teleport, would remain unloaded, and not a concern for the server.   If a player telports in or approaches on foot, those chunks will then reload and the mobs slowly despawn.

Honestly, other than my brain fart yesterday, play testing has been going well. I haven't noticed anything odd with passive mobs anywhere, so I think number 1 on the to do list may not need doing. I would still sort of like it if a mob could follow you into the area though. So I'll have to wrap my head around that one of these days. 

I still haven't noticed any unusual lag at any time other than if I force kill all the hidden mobs. Then it stutters for about half a second as more mobs are loaded and replaced. Something like that should only ever happen when a scarecrow is first placed though, so I'm not too worried.