Personally, I don't think this theory will come true.
Percy is one who strives for order. Legal order. The reason he was against Harry in OotP is because the MINISTRY was against Harry in OotP. I see him basically as a Ministry underling that will do anything (within the rules) to get ahead. We saw him at Hogwarts studying, studying, studying and being an absolute stickler for any rule of any kind. He actually thought that those were the ways he was going to get ahead in life.
He knows the right side - he just is under the impression that whatever the government thinks is right, and absolutely right with no exceptions. We saw how he reacted in HBP; it's hard to own up to a mistake as large as the one Percy made, especially the kind of mistake Percy made, and then have to go back to his family, knowing they were all right and that he was the one that was wrong. With Percy's personality, that would just grate on him to no end. He would not be able to admit it.
In short, I don't think it's possible for Percy to be a Death Eater. He's just so determined to make it to the top of the government that he doesn't really use his common sense that much - just his political sense. And his pride, as we have seen, is very easily wounded and not easily mended.
And, as Sirius (the character 🙂) mentioned, "There are more people in the world than good people and Death Eaters." *paraphrased, don't have my book. 🙂)
At least, these are all my opinions. Take them as you will, Nickey. It's certainly not hard to see where you got to this theory. I think you just have to keep in mind that Percy DOES know the good side, it's just that when he thinks he's right about something, and he's not... well, his pride is a big thing with him. . I see him "coming back into the family fold" in some shape or another, extremely awkwardly of course because most of his siblings think he's a git, but I see it happening, in DH. I can also see something coming out of "pride comes before a fall," and Percy possibly being wounded by Death Eaters; that would certainly bring him back into the numerous arms of the Weasleys, I think.