I also highly doubt the “limelight” argument. So far every dlc character has had a dedicated presentation highlighting the character, as well as their full gaming history. So no matter what, every dlc gets the limelight of a presentation.
I'm not sure you're understanding the point: it's not about focus, it's about exposure. A Smash Direct is watched by, well, Smash fans. A Nintendo Direct is watched by everyone interested in Nintendo, and thus gets substantially more viewership. If you're a third party lending your character to Smash, it only makes sense to want as many people to hear about that as possible, not simply those already looking for Smash characters. I do agree it would be somewhat surprising for a third party originally planned for E3 to have stayed a secret for three extra months, but leaks aren't inevitable (IIRC Joker stayed secret, for instance), and I do believe a major third-party would help explain why a Smash announcement has been delayed this long instead of simply doing a dedicated FP7 reveal stream.
Kind of a random aside, but there's one bit of recent non-Smash information that has guided my thought process of late. I've long been on the mailing for Evergate, one of the games that was released after last month's Indie World Showcase. When they sent out a message to fans shortly after the presentation, they noted that they had been selected to be part of that presentation in June, roughly
two months prior. That also implies that they had been discussing a potential Direct slot, as well as the possibility of launching alongside said Direct, even earlier. None of this is necessarily surprising, but it confirmed what I think we all ought to expect: the contents of these Directs are planned well in advance, and their importance is such that even release dates can be affected by them. No wonder, then, that FP7 has remained a mystery for so long when it got (presumably) bumped from an E3 reveal to, at best, a September Direct reveal. Maybe it's because it's tied to a game reveal (as we're certainly hoping), or maybe it's a third party character, or maybe Nintendo simply wants to ensure Smash fans will keep watching every Direct. Whatever the reason, it seems unlikely we'll hear about FP7 anywhere other than a major Direct at this rate, for better or worse.