Hidden Requirement for Rival Events and Returns

In my own game, I had no trouble whatsoever triggering the Rival Events at the earliest opportunity with respect to the 1st, 2nd and 3rd Rival Heart Events. The
4th Rival Heart Event for Pierre and Natalie triggered at the start of the 4th Year and on the same day, I was able to trigger the 4th Rival Event for Elliot and Julia.
The Event I could not trigger as easily was Denny's 4th Rival Heart Event.

I've written about this in another post. It was when I made an effort to fish again and caught a certain number of items that the Event finally triggered.

Vaughn's Rival Heart Events could not trigger as I married him myself, incidentally...

I thought it was only the 4th Rival Heart Events that had this requirement until other players informed me that they could not trigger the 3rd Rival Heart Events.
It appears that every Rival Heart Event shares the same hidden requirement. For Denny and Lanna, it is fish. For Elliot and Julia, it is Crops. For Pierre and Natalie, it is Cooked Dishes and for Vaughn and Sabrina, it is Ores.

Now I am in the situation where the Rivals have gone off to Mineral Town with their spouses to have their babies. Pierre returned the very next day but neither Denny nor Elliot returned as quickly.

The Rivals went to Mineral Town in Winter. I believe that the reason why Elliot could not return perhaps may be a requirement to ship the crops for the current season. I shipped many Crops during Winter but they were out-of-season Greenhouse Crops. I expect his return will be announced when I have been able to ship a specific number of Spring Crops in Spring.

Denny's return was announced as soon as I shipped a specific number of items in Spring.

The requirement ordinarily is a total of 30 and I expect that is the total in these circumstances as well.

In fact, it really does not matter if the 'husbands' return early or not. One full season after their departure for Mineral Town, each Rival Couple will return with their child. An early return of the husband can be managed by meeting specific requirements but even if you do nothing to facilitate the return, the entire family will return a season after their departure.

The date actually should coincide with the anniversary of their Wedding a year previously.

If you cannot experience one of Elliot and Julia's Rival Heart Events, ship 30 of the current season's Crops.

If you cannot experience one of Lanna and Denny's Rival Heart Events, ship 30 Fish.

If you cannot experience one of Sabrina and Vaughn's Rival Heart Events, ship 30 Ore.

If you cannot experience one of Natalie and Pierre's Rival Heart Events, ship 30 Cooked Dishes or at least cook them.

That is the one variable I have not confirmed: do the items actually need to be shipped?

Note that I had not ignored the husbands: they were at 10 Hearts and they left only when their wives went to Mineral Town to have their babies. I expect that the requirement for their return IS the same, however, as when you have neglected them, forcing them to leave. As previously stated, however, it is merely an odd little quirk of IoH, as the entire family ultimately returns a full season after the departure of the Rival Couple, accompanied by their child.