You know with all this joke about Takeda being Aoyama from Earth-2. I have my own joke theory as well.
Have you ever wondered why their dad secretary, Ebate is so loyal to him? Well, that’s because the quint’s father and his secretary... ARE ONE AND THE SAME!!!
They both have the same first name, Maruo (ch.50 p04 and ch.68 p14). Even they're hairstyle is similar to each other. When he first arrive at the quint home in ch.49, Itsuki brings up that despite all his works throughout the years, this could be his first time he's ever come into their home, but Ebate never answer that question and just commented that they're still children in his eyes. So, Ebate definitely seems to know more than he let on.
My take is that somewhere in the distant future, a older Mr Nakano somehow ended up in the past. From there, he change his surname but not his first name, as to remember who he was. Then enrol in a university before becoming a school teacher. Somewhere down the line, he met his younger self and becomes his employee. He then watched the development of his past self, the quint and eventually Fuutarou from a distance over the years as a passive observer.
Fast forward to December, 2018. Knowing that his younger self would encounter Fuutarou's dad in the coming spring. As to prevent the timeline from altering because Mr Uesugi may or may not sees the similarity between them, he took the initiative to overhaul his entire appearance as to prevent Mr Uesugi from ever recognising him.
But somewhere inbetween now and the wedding, he manage to somehow find a way back to his future. That's why we don't see him at the wedding five years later.
(Note: this is all a joke and by no means should be taken seriously)