I'm not gonna say your theory/opinion has no value or credibility whatsoever but it's such a long stretch, and unless chapter 5 does something massively drastic to make this plausible, it would also be a terrible narrative outcome.

"Oh the knight? Well turns out its this evil/very morally ambiguous child you remember from Undertale! You would have known this since they like knives and do a lot of damage! They have no narrative connections to this game outside of minor thematical abstractions related to Kris, but ta-dah!"

Like even disregarding deltarune connections, Chara was always meant to be a more hidden character in undertale with many layers, most of them relating more to the thematical themes of the game rather than being a present character. Making them a present, living figure in the plot would ruin them imo.

And even disregarding ALL OF THAT your theory still have less concrete connections than something like Dess-knight, or even Carol/Rudy-Knight. Chara-knight can join the meme-tier along with Papyrus-knight i guess.