If delays do come up due to COVID-19, I could see them re-negotiating to move some 3rd party characters to a later slot, as Nintendo is more in-house and easier to travel to.
It however has shown no signs of that being needed. But we still have a year and a half. Things could change. But yeah, there's no real evidence this has happened, and definitely none for cancellations.
Nothing wrong with being pessimistic about the idea that "the exact order will stay the same" and all. With such a deadly pandemic, worrying a little is reasonable. But cancellations are a far bigger deal than changing the date of a release. Both would require new negotiations, but companies are releasing stuff later too, so they're pretty understanding in the regard of delays. Spring Man alone was delayed, and that could've affected any of the other 5 release dates, even if just a little. Likewise, the pandemic came up after they already figured out all the characters as is. That means if delays happen, they wouldn't have had an idea set up of who to change around by default. Of course there'd be new negotiations even for delays. Delays matter for a company, as the timing of a reveal and release often coincides with a game they have coming out. A promotional effect matters. That could mean they might not want to delay their game, but they may move it up a week or so depending the situation.
It can get pretty complicated, yeah. But there may have been ideal time periods for each character release too, ones that are extremely flexible. We don't know, and may never know at any point. But I do think the idea some characters could be switched around isn't that farfetched. Not really ARMS, as it was likely the pass opener as is, However, didn't Sakurai say they had to delay the release? I don't think it's cause of COVID-19, but more because extendable arms require extra balancing.