I guess I'll put in my two cents on who the fighter likely is, as well as how Spirits, Mii Costumes, and Assist Trophies figure in.
I'm fairly confident that at this point that base-game Spirits do not de-confirm. For FP2, which as far as we know was decided after the development of the main game was completed, it seems to me that all the popular characters who were initially Spirits would be back on the table. The developers wouldn't lock themselves out of using the Spirit characters now that they have a chance to make more fighters, especially if they're going to make more first party choices. So Min Min, Ninjara, and Twintelle are from that angle viable choices.
As for the "extendable arms" comment in regards to Twintelle, however, I'm not sure, but feel it hurts her chances. It seems likely that whoever wrote that knows something about the character in order to make such a statement, and if they said arms, they probably don't have Twintelle in mind. I'd say there is a small chance of this not being true, but I'm not holding out much hope for her after hearing that.
As far as Mii Costumes, I feel like they likely don't de-confirm either. We have plenty of Mii Costumes of playable characters, including some of fighters that have been there from the beginning, like Samus and Fox. This suggests that the developers don't see Mii Costumes and fighters as being mutually exclusive in the first place. It is probably true that the Spring Man and Ribbon Girl costumes were added as a recompense for not being included, and in my mind, that puts them in a similar boat as the base-game Spirits. IMO, it wouldn't be any more unlikely that the developers would add a Mii Costume character as a fighter after the fact than it would be for them to add a Spirit character (as such, I feel like Rex is also now a viable choice, but that's another discussion altogether). What's more, it could be argued that the fact that they have costumes of Spring Man and Ribbon Girl bodes well for them, as they're an implicit admission that they're the go-to characters for a fighter. That said, however, I'm personally not sure it will be Spring Man, for reasons I will get to shortly.
The idea of an AT upgrade does give me pause, although I admit I could definitely be wrong in this case. True, there have been AT upgrades, but that was a game later when they had the opportunity to remove the AT. Having base color Spring Man appear in the same stage as the Spring Man AT could be seen as a design flaw, and if Spring Man is the choice, the devs need to figure out what to do. I don't see them doing something as in depth as changing the AT to Springtron, but I suppose removing the AT (probably altogether) could be an option. Of course, all this could be avoided if they chose another character, and in addition to being less complicated, such a move could also be more spicy and original.
Of course, the perception that Spring Man is ARMS' go-to rep would take issue with any other choice, but I nevertheless feel it doesn't mean he's a lock either. If he is the Smash character, that doesn't explain to me why they are building up to the reveal. It seems as though they would just say so if he is the choice, as opposed to showing the whole roster as possibilities at the recent update. They seem to want the leave the question open, and choosing Spring Man alone after getting everyone's hopes up for their favorite seems like a clumsy move. The idea of having Spring Man in addition to skins of other fighters, however, would be something I could see them doing. I'm sure they would enjoy catering to multiple people's hopes by revealing a Hero-esque four part fighter.
So, we have a number of different possibilities, but in my view the most likely choice is either A) a four-way character like Hero including Spring Man, or B) a single fighter other than Spring Man. The question remains, then, who? Well, I personally don't see a better choice objectively than Min Min, the super popular winner of the Party Crash Bash. Ribbon Girl is another possibility, but since Spring Man is the face of the game even over her, I don't see why she is any more likely to be chosen over the other popular characters. I personally would have liked to see Twintelle, but I don't see things going in her favor over Min Min. And I'm fine with that, really-- pretty much any choice would be a win in my book.
So, I'm just holding out to see who it is! Once we know who, we'll have a bit more information regarding what does and does not "de-confirm", and from there, we can begin to speculate regarding the remainder of FP2. Personally, I'm psyched!