That's a good theory too.
I had another one coming to my mind too.
Maybe you have to believe in finding love, to have a red string. Since Kaoru always thought, that there lovers would leave him behind, once they find their fated ones, maybe he didn't. So maybe it will only grow now, when he starts to believe in Hara and his nephew will just be the first to realise it.
I really wish for the new chapter to appear soon, so that we can read how it's actually going to be. All so theorys make so excited.
I believe it will be fate vs choice, Hara-san still can choose to be with him even if they are not fated partners
Hmm
*Hmm It's possible and I wish too
Kaoru's ability available see green and red fate string. Kenji's ability available see red fate string. Maybe Kaoru's string different color? Maybe that's why Kaoru and Kenji don't see Kaoru's fate string. Maybe Kaoru's nephew available see Kaoru's different color fate string? (=・ω・=)
[I'm sorry my English is not good enough to explain it so if I make a mistake please forgive me ( T﹏T )]