Anime

Tbf, how the fuck do you explain all this? Not saying you're wrong about needing to be open to spouses, but Rudy's situation is damned messy. From Sylphie and Roxy's perspective, there's just a random dead guy that shows up in the house in the middle of the night. That alone is suspicious. And given that Oldeus was already dead by that point, they only have Rudy's account of what was said. If Rudy just explained what was said to them and they just accepted it without question it'd be unrealistic.

Also, I don't think they're aware that Rudy was reincarnated and that he was in his 30's when he died in his previous life. This detail has huge implications on how he's interacted with them since they first met. And Oldeus knowing Rudy's original name is the main proof behind him being from the future at that point, as Rudy hadn't frozen/found the rat yet.

Rudy's reincarnation would need to be revealed to explain everything. While Cliff can probably see it with his new demon eye he's unlikely to leak that detail given how much they've been through. The only other way it could be revealed is if the Man-God tells someone else. Honestly, keeping his future self (and by extension his reincarnation) secret is the simplest solution. I get that keeping secrets from your spouse is bad, but his actions here are understandable given the lopsided risks between being open and being secret.

The big issue is how does he restart contact with Eris? Just sending a letter out of the blue is suspicious (though it's unlikely Eris will pick up on it) and Rudy still has mixed feelings about her. The other options are to try to engineer a more natural seeming situation for them to meet up (which isn't good due to the manipulation involved) or to just wait for Eris to show up on her own and hope things go smoothly.

That being said, in that last option I listed it'd probably go better than it did for Oldeus since Rudy won't be grieving for Roxie and Sylphie and also he knows that Eris's aggression is from her caring for him and being terrible at communication. Personally, I think letting Eris just show up on her own and letting things work out naturally would be the least manipulative option at this point. But doing so is a risk, and with Sylphie's and Roxy's safety already being threatened Rudy's probably not willing to take chances

Self report on the author for how relationships work

It's a very difficult spot, because of the butterfly effect. I suppose some choices would become very hard if his wives would also know of the diary. Taking Eris in, for example, would not be easier if they knew that it was mostly to avoid in-fighting, or further difficult paths, and not love.

I would guess he has to make her feel at home in the new world, that's all he can offer. She has to come with acceptance that too much time would have passed, and everyone she knows is dead. And he has to offer her a place to belong.