My feeling is they probably knew since some point last year that the console not releasing this year was possible/likely and so they began preparing for about nine months to a year additionally where the Nintendo Switch will still be the main focus for marketing. I do feel like, as you said, it was probably always the plan to have a Partner Showcase this month and by the time it was solidified that the console wouldn't make this year, it was easier to begin planning for another Direct opposed to reworking this one and possibly having it shifted into March, probably messing with the plans of their partners? Using this thinking, the Endless Ocean project was probably always going to be in the Partner Showcase, I just feel like it carries a different meaning now!
Before the reports, I had felt like we'd get this Partner Showcase, the console reveal in the form of a First Look or larger press release, and then a smaller Mini in late March/Early April for the Nintendo Switch that would date Paper Mario for July and announce Fire Emblem 4 Remake for August as the two headlining announcements. That would've been followed by a larger June event and a (General) Nintendo Direct that would highlight Nintendo Switch 1 games and Nintendo's main Next Generation games for this year in July.
Now I feel like they probably do convert that Mini in late March/Early April (probably early April now) to a General, having the two previous announcements but then maybe pulling a game or two that would've been in the July event (if I had to guess, something like a Mario Party and something related to Metroid) and feature them here while then having the June/July Direct still that highlights the same games they might've planned with an additional one or two that they may slot in.
I feel like this response is all over the place so I'm so sorry!! My main question I feel is the range that an April General would cover? Normally with the February Direct (and even with this Partner Showcase), we see that they focus on the first half of the year but I feel like April would be a little late to focus on just the first half. Maybe it would focus on the general range of "games releasing this Summer" like they use the "Winter" for the September Directs? Then whatever they do in the middle of the year would be focused on what is releasing through the rest of 2024?