So I've been playing this world singleplayer/lan with my g/f while I've been streaming for the last few weeks. This was a newly created world specifically to stream on so its really no that old. Here are my current issues with the game that I've encountered that have been incredibly frustrating.
1. Passive mobs spawn within 16 blocks behind me randomly.
2. Old school mob platform/traps dont seem to work. Streamed yesterday demonstrating, mobs simply dont spawn. I built 2 different types, one that I've used multiple times for skyblock modpacks in Java Edition up near the sky limit. The other the giant room, if you skip around the middle of the following video you can see me building these structures. Even tried creating a new world thinking it had been fixed in a newer update and it didn't seem to fix it. https://www.twitch.tv/videos/160488058
3. Also as seen above in the video there was a wierd lighting glitch, fully dark room, strange light making it fully lit still. No explanation. About the 2hr mark of the video.
4. Rain falls through solid blocks near the sky limit.
5. I've not seen a single squid spawn.
6. Wither skeletons can spawn on halfslabs.
7. Halfslabs let light shine through them.
8. In a singleplayer game only things I've named with a nametag keep their name. My girlfriend who connects via lan has had to rename her donkey 3 different times and if I close out the game it loses the name.
9. My current world, I've not seen any chests in my nether fortress, a newly created world however does have them so possibly a bug thats been fixed since my world was created.
I'm sure there are more I'm not thinking about but number 2 really baffles me. Especially if things are built near the sky limit stuff should still be allowed to spawn. I've seen videos of people making mob spawners and such but I have no idea what the are doing different. Do I still need to clear out all caves even at the sky limit?
Win10 is not the java edition. It may look like it (for now), but it doesn't act like it, nor is it suppose to. It is it's own game. If you're thinking the two should be exactly alike, you'll always be disappointed. Stick to Win10 videos, things work different than in java version videos.
If you think there's bugs in the program (like #4), you need to report them on Mojang's bug site for them to get fixed. Some things are not bugs: I've seen plenty of squid (#5). I use the Natural pack, which lets you see clear to the ocean bottom. I've seen dozens and dozens of squid.
1. No mobs should spawn within 24 blocks of you. If they're spawning behind you, how sure are you they didn't just wander close after spawning?
2. and 3. I watched a good part of your video. There were several issues, primarily that sometimes you used slabs (which are transparent in PE/Win10E) and that your spawn floors were small.
Spawn floor efficiency depends on size. In your first try, the floor was only 15 blocks in area. When the game tries to spawn, it picks a random block anywhere within the chunk, which has a volume of 65,536 blocks, so the chances of it picking one of your blocks is only 15 / 65,536 each time. Hostile mob spawn attempts happen every game tick, so your chances come out to about 27% per minute, meaning you'll have to wait about 2 minutes on average between spawns, and of those there's only a small chance of it being an Enderman. A bigger floor would increase that proportionally. However,
Sometimes your spawn floor was only 1 block wide. I'm not sure, but I think once the game finds a spawnable block it tries to spawn a mob at a random point within that block. This means there's a good chance the mob's hitbox would intersect that of a wall next to it, canceling the spawn attempt. I experimented with this setup and couldn't get any spawns with a 1-block-wide floor, but I did at 3 blocks wide. So again, you need a bigger floor. Furthermore,
The area where you wait is about 43 blocks from the spawn floor. The range is 24 to 128, but if you're more than 32 blocks away any mobs that spawn will despawn within 27 seconds on average. Any mobs that do spawn thus have a pretty good chance of despawning before the next one spawns. You probably would never see more than 1 mob at a time, ever.
You probably aren't having problems with the mob cap. Endermen are neutral mobs, which have their own mob cap of 10. Unless there are 5 or more neutral mobs (most likely spiders in bright light) elsewhere in the chunk, the cap shouldn't affect you much. If you were farming for hostile mobs, yes, you would need to light up all the caves, and that could be another reason you're not even getting hostiles spawning here let alone Endermen.
You probably set up your spawn floor this way because you've seen it in Enderman farms. What you don't seem to have realized is that those farms only work well in the End, where Endermen spawn a lot and other kinds of mobs don't spawn at all. Endermen are rare in the Overworld, so farms there will spawn mostly zombies, skeletons, spiders, and creepers with only an occasional Enderman. Which brings me to a suggestion...
You can get just as many ender pearls by simply spending the night in a fairly level desert. Build yourself a 1-block high wall 7 blocks on a side with a roof 2 blocks above it. Stand in the middle and watch for Endermen. You'll see them easily since the desert has few obstructions. When you see one, look at it to provoke it. It can't reach you through the opening and can't teleport under the ceiling, so you can kill it safely as it tries to attack. You're also safe from zombies, husks, and creepers (while you're in the center) and can pick them off for bonus XP. (Skeletons and spiders will still be threats though, so you'll need to keep an eye out for them.) You'll probably get a few Endermen a night, and if you have a decent sword you can kill them before they give up attacking.
4. I couldn't reproduce the rain problem at Y=250. Was it actual rain, or could it have been the water drips you get in PE when there's water 2 blocks above you? Rain shouldn't cause that, though.
5. Squids in Win10E only spawn in Ocean or Deep Ocean at Y=46 to 62, so you'll only see them if they happen to wander upward or you go underwater. I had to go diving before I saw my first one. Later I found some close enough to see from above, but not actually on the surface. I think this is a difference from Java Edition.
6. Anything can spawn on a top slab. Are you saying a Wither Skeleton spawned on a bottom slab? That shouldn't be possible.
7. Transparency of slabs and stairs is an exclusive feature of PE/Win10E.
8. PE/Win10E has a bug with nametags. Your experience is quite common. Hopefully 1.2 will fix it.
9. There have been occasional reports of Nether Fortresses generating without chests. It might still be a bug, or maybe it's just random chance.
Grass doesn't pop up out of nowhere, it has to spread from an adjacent block, so if you have too many sheep in one place you get large patches of dirt in the middle that never seem to grow grass again because most of the grass blocks are too far away to reach them before the sheep eat it. The only way to avoid that is to give the sheep a LOT of space, something like 15 blocks for every sheep. You'll still have some dirt patches in the middle, but there will be enough grass along the edges to keep them growing wool. BTW, you do know that sheep won't die or despawn if they run out of food, right? They just won't regrow their wool after you shear them.
Your sheep problem can be solved by a larger field, divide it in two with a fence and a gate (preferably a double gate) in the middle, and keep the sheep in one half at a time. When the grass is all eaten up, move them to the other half (they'll all follow you if you hold some wheat), then close the gate let the grass grow back on the first side. You could even expand on that idea and have an even larger field divided into quarters, since grass takes a bit of time to grow back and the sheep eat it too quickly!