Corrected entry: The whole reason Piper and Leo can't be together is because he has become an elder, but he's present at home just as much as he was when he was a whitelighter. And why would he be worried about "breaking the rules" now after all they've already been through?
Corrected entry: Throughout season 6 Chris continually refers to his memories of the Charmed ones in the future (using the words "the three of you", yet in "Oh My Goddess" he tells Phoebe that if he hadn't come back, Paige would have been killed that day. So how can he have memories of the three of them when Paige died before he was born?
Correction: You've answered your own question: "if he hadn't come back, Paige would have been killed that day." He came back, so she wasn't killed that day, so he has memories of her from the future.
Corrected entry: Why does Chris have his powers when he went to the past if the Charmed Ones never had their powers when they went to the past, for example in "That 70's Episode" or "Witchstock?" If Chris came from the future and traveled to the past, shouldn't he have lost his powers too?
Correction: Bianca states in Chris-Crossed, that with the particular spell that they were using to send him back, his powers would not be affected.
Correction: Maybe in the future they have come up with a spell that allows them to go back in time without losing their powers? It isn't impossible to believe they have improved upon their spells.
Corrected entry: Why doesn't Chris ever use his powers against demons? We know he can use them in this time, since he blows up Leo in "Oh My Goddess" and kills a Valkyrie in "Valhalley of the Dolls", and after "Chris-Crossed", when it was discovered he was half-witch he had no reason to hide them from the sisters, yet he never once used them against a demon, even when one of the Charmed ones is in mortal danger.
Correction: Actually Chris couldn't use his powers up until the "Chris-Crossed" episode because before this episode the sisters couldn't find out that he was part witch because then his cover as just their whitelighter would have been blown. Basically he had to keep his powers a secret until the "Chris-Crossed" episode.
Correction: Leo tells Piper that the other elders want him to stay on earth until he finds the one who banished him to Valhalla, and Piper knows eventualy he will have to return to the elders.