Sakurai has *full* control of the roster. Even TPC basically asks him to *pick* a Pokemon, or heavily *implies* one they'd like him to add.
And unfortunately for Rex and Pyra, their designs weren't even finalized until mid 2016. Elma was fresh in everyone's minds. If Sakurai picked a Xenoblade character for the base roster, it's Elma. Full stop.
Arms is a different case, as it's an entirely new franchise. Sakurai could, as he has in the past, not trust it on its own two legs yet to just add it, but again, it probably missed the boat.
OK, c'mon. Shulk is *not* an advertising character. Do you *know* how popular Shulk was in Japan? And regardless, Xenoblade was 4 years old by the time of Smash 4's release. Sakurai likely added Shulk because he saw uniqueness potential and liked the character. We know he's a big Xenoblade fan. You really think the "marketing base" went out to buy an excellent but extremely niche genre game because Shulk was in Smash?
Lucario would definitely have been added. He's the new-gen (at the time) mascot, and was incredibly popular, especially around here. Lucario showing up was no surprise. He was everywhere, and even usurped someone like Mewtwo for a while in the eyes of younger fans who grew up with him.
Wii Fit is not advertisement. Wii Fit U had already been out for ages, and the series is incredibly dead. Sakurai literally picked her because she thought no one would expect it and she was unique. He even said this.
"Every Fire Emblem character" I don't think you're giving Fire Emblem enough credit. It's become very popular (mostly with the Switch's target demographic, 20-30 year olds), and even in the Brawl days, Ike had a very vocal fanbase before he was revealed. Robin and Lucina are huge, Lucina even being so popular IS pushes for her inclusion in basically everything. Corrin, sure, but that's DLC.
I think you're overselling how important "marketing" is to Sakurai. Sakurai takes characters and series people are unfamiliar with, and MAKES them bigger. If K. Rool appears, he's going to be relevant again. Probably cemented back into his series forever. You see it with all sorts of characters in Smash. Especially the Fire Emblem ones.
Celica and Impa are relative nobodies when it comes to fanservice. The "kids" demographic don't play Fire Emblem, especially not a game like SoV. They can make it, but Celica's game was a quickly-made remake Sakurai likely wasn't aware of, as it wasn't even announced until January 2017.
Oh, and about Ridley, no, Ridley was gone. Other M was in 2010, and Samus Returns, like SoV, was a remake made relatively quickly that Sakurai probably didn't know about. It had a mid-late 2017 release date and likely had no impact on Ridley's inclusion.
If this game is about fanservice, cater to the fans. The Smash fans. Kids, and everyone else, don't care nearly as much, and will probably be introduced to stuff they never knew about through Smash and enjoy it. It's kind of what Smash does. I'm certain most of the kids you work with don't know Ridley through Metroid, but have probably seen him in Smash. Playable Ridley will probably garner interest in the series.
Welcome aboard!! Glad to have you! Here's hoping Ridley and K. Rool can finally get their day together after all this time!