No, slimes can't spawn in the sky.
According to the decompiled Slime Entity code, slimes can spawn in swamps only between the heights of y>50 and y<70. That means a spawning pad 5 blocks below the build limit is far too high-altitude for slimes to spawn.
If Endermen take any form of environmental damage (fire, water/rain), they teleport away and lose all interest in everyone who might have hit them before. so if you hit them with a fire sword, they will start burning and immediately teleport randomly for a few times until the fire goes out. If no environmental damage is dealt, they will pursuit you until they kill you or you run out of range.
Enderman as well have the habit to stand still then teleport if you stare into their eyes then lose contact. This can cause them to teleport around frequently during battle, so try to look down at their feet. In addition the teleport is in all directions, so they could be above you or below you in a cave after a teleport, unable to reach you until you get out of range.
The easiest way to kill an Enderman is to use their size against them. Build a 3x3 ceiling at 2 blocks height so that you barely fit under it. Then pull an Enderman to this location and he will keep running against the ceiling, unable to hit you and unable to look at you, while you can easily finish him off. You can as well use natural ceilings (leaves, rocks) at the same height instead of building one. Use a looting sword for increased chance to drop.
If you have any form of teleportation mod installed, you can do the same in the End world, which will thanks to the huge amount of Enderman gain you pearls very quickly. If the dragon is still alive, things get a lot more difficult. Then you need to find a location where the dragon is out of range. In some End worlds this is possible, in others the island is just too small for that.
Best Answer
Since the only items impossible to obtain in peaceful are mob drops (that are not obtained otherwise) and derivatives, here's a list of unobtainable items:
There are only a handful of direct derivatives:
And here is the list of drops you can still obtain:
It is also to note that a lot of items, that are possible to obtain, won't be renewable anymore, like: