Sidious told Vader to go to Mustafar to end the Seperatist movement, but did he know that Obi-Wan would show up and defeat Vader based on his "vision" of future events? Was this Sidious' way of ensuring his power by encumbering (Vader's suit) his only true rival?
Here’s a quote from the Revenge of the Sith script, just after his duel with Yoda:
CLONE COMMANDER THIRE: There is no sign of his body, sir.
MAS AMEDDA: Then he is not dead.
PALPATINE: Double your search.
CLONE COMMANDER THIRE: Yes sir. Right away, sir.
PALPATINE: (to Mas Amedda) Tell Captain Kagi to prepare my shuttle for immediate takeoff.
MAS AMEDDA: Yes, Master.
PALPATINE: I sense Lord Vader is in danger.
This menace to his apprentice is new for him, so he cannot have planned it.
As a complement, here a quote from Palpatine’s Wookieepedia article, specifically the Transformation of Vader section, which cites Star Wars Episode III: Revenge of the Sith (junior novelization) (emphasis is mine):
The battle between Kenobi and Vader had ended much differently. Kenobi had succeeded in grievously wounding his former apprentice, leaving him for dead. Hurrying to Mustafar with speed only the Emperor of the galaxy could afford (and with vigor that surprised even himself), Palpatine found his apprentice badly burned but alive. Sidious finds Vader's body scorched by lava after losing to Obi-Wan Kenobi on Mustafar.
Even as he made his way to the body, Sidious wrestled with anger and indecision. Part of him wanted to leave what was left of Vader to burn to ashes in the rising lava; even if he had survived, his body would be crippled, as would be his powers. Even the dark side of the Force required living beings to generate it, and too much of Vader's living flesh would have to be sacrificed in the act of saving him. Whatever the outcome, Vader would no longer be the perfect being to continue the Sith legacy, no longer be able to fulfill his promise. However, hobbled as he was, Vader would still be powerful, and there were no more Jedi to oppose him. So Sidious kept walking and, finally reaching his apprentice, placed a hand on Vader's forehead, using the dark side to keep him stable, while his shock troopers prepared a medical capsule for the return to Coruscant.
Vader's trip to Mustafar was twofold: 1) To clean up loose ends with the separatists and 2) to solidify Anakin's loyalty to the Empire and the dark side.
Sidious wanted Anakin to be the one to kill the separatists; this would fuel his hate, rage, ego, etc. He also wanted Anakin to fight Obi-Wan. As long as Anakin survived the encounter it served Sidious's purpose. If Anakin killed Obi-Wan, then it would send him barreling towards the dark side in a flood of hate and self-loathing. If Obi-Wan escaped, the encounter would still drive a large wedge between Anakin and remnants of the Jedi order thereby leaving him no where to turn except to Sidious.
I don't think Sidious expected Obi-Wan to best Anakin and his distress was genuine as Anakin was to be Sidious's most promising apprentice.
No, I didn't get the impression that he did. During Sidious' battle with Yoda, he told him: "Darth Vader will become more powerful than either of us! He sent him to Mustafar so that Vader could be submersed even more in the dark side. After Vader kills the Separatist leaders, his eyes glow yellow (a by-product of using the dark side of the Force).
Before I get into this I just want to state that I'm going off what is shown in the movies and not the books (which I have read and thoroughly enjoyed) because there are many people who have not read any or many of the EU material. So, out of fairness, I'm basing my argument on the films. In the end, however, it is just my opinion.
Palpatine is an opportunist and has the ability to see many possible futures, but that doesn't mean that nothing can surprise him. I believe that he genuinely expected Vader to defeat someone like Obi-Wan (I don't think the fight was part of his plan at all, what with Order 66 and all) with relative ease because he was younger and more powerful in the force than his former master and many if not all the other Jedi. Of course, Vader was handicapped by a couple things.
1.) Arrogance: He assumed that because he was more powerful he had nothing to fear and underestimated Obi-Wan.
2.) Naivety/Ignorance: He assumed that power was more important that wisdom. He didn't pick up on the ridiculous number of clues that Palpy threw at him in ROTS. I mean, it was like he wanted Anakin to figure out that he was a Sith but finally had to be blunt cause the guy wasn't picking up on it. Granted, subtlety was never Anakin's strong suit but still...
Going from those two things, it's not that surprising that he was defeated, but Palpatine probably wasn't expecting Obi-Wan to show up and even if he was he probably figured he'd be unwilling and therefore unable to kill his friend.
I'm also going to assume that in the end he didn't care too much about having Vader around. If he lives, control him. If he dies, good riddance to a potential rival. Betrayal and manipulation are core tenants of Sith philosophy.