Emperor Revan
Most Powerful Sith Lord
Again, why did he need Sion if he could just eat Kreia? There's no point. We've never seen him do it in combat. And why does Sion say "You are difficult to kill." to Kreia? As for her quote, she couldn't resist Malachor V either, Revan could. She didn't think it was possible to hear Bao Dur's thoughts either but the Exile did.
3. Ok you're probably right about the wound, but what about the last part I said, do you really think they could've boarded his ship, fought their way to the deck, and through Visas on their own? Very unlikely. They were probably just near and Nihilus drained their Force and they couldn't pilot their ships and hit a wall or something.
4. They continually say he went off to fight the Sith Empire, and have made such a big deal about it that they're not gonna say "oh it was all a lie" or something like that now. As for the unfinished game, it's not in the game, and if in the 'original' version he didn't necessarily send Kreia to do anything. Even if he did, think of that sentence right there, Kreia being told by Revan to do something and her doing it. Sure she liked Revan but she wouldn't just say "ok, I'll go to Malachor V, fall to the dark side, train this guy and blah blah blah" just cuz he said so, especially with her hating the Force. She would have to be really scared of him for her to do something like that. Even if Revan did send Kreia to kill Nihilus, he probably was busy and couldn't do it himself. Like I said, Nihilus is stupid and Revan is a brilliant tactician that can turn the tide of most any battle. If he really wanted Nihilus dead he would've done it himself in one way or another. And why send Kreia to train the Exile anyway? The Exile followed Revan before and the Exile respected him so Revan could just train the Exile himself right?
I really don't think Nihilus can use it right in combat and then walk up to someone and kill them or he would've done it to Kreia singlehandedly and then done it to Sion. I really think he has to concentrate on it and that would be hard while fighting. Besides, Lord Revan knows dozens of techniques to kill or turn Jedi/Sith. He used the one that the Exile did to Sion frequently for instance.
Now as for the reason the Exile could beat him, I really do like the idea but it's not a fact that he doesn't have the Force, he just uses it from others. Look at when he goes up to Nihilus, Kreia is far away, and if there are any other Force users they are down on the planet. The only person he would have to draw the Force from would be Visas and then wouldn't he only be as powerful as her? Couldn't Nihilus simply use his "eating" thing on her and the Exile would be powerless? Now granted he has a chain with Kreia and probably gets his power through her, but then couldn't Nihilus just do that to her through his chain? Couldn't he eat the Force flowing to him from her? They're supposedly like the same person and he did it to Kreia once anyway. Why wouldn't he do it to Visas either? My answer to all these questions: He can't do it to someone powerful enough. Kreia most likely got much stronger from when she was first Darth Traya and he had to have Sion help him anyway (Kreia could even move her lightsaber a bit with the Force still) and the Exile would be too strong to use his technique on as well. Since Revan is more powerful than both these two, with a much stronger Will, is far more strategic, and has more knowledge about the Sith, I'd say it's unlikely at best that Nihilus could use his technique and kill Revan.