What happens when I create a Nether portal in the End?

As has already been said, you cannot build a portal in the End.

However, they can still be hacked in and used. Using creative mode and INVEdit, I located a naturally spawning End portal frame, and traveled to the End. From there, I placed a portal block and traveled to the Nether. Finally, I placed another portal block and was spawned back on the Overworld.

End Portal: (-1007,16,-123);
End Spawn: (100,53,0);
Nether Portal: (89,64,-21);
Nether Spawn: (11,62,-3);
Overworld Portal: (8,46,20);
Overworld Spawn: (74,64,163);

Note than when I spawned in the Nether, I was inside other blocks and no Overworld portal spawned with me. Spawning back to the overworld worked fine.

Nether portals created in The End will simply not activate when you ignite them.

Response to New Edit

I have no clue, but I would suggest backing up your world before trying it. It is possible that you won't be able to go through the portal and you will just stand there.

you will have an obsidian frame with fire inside.