I've been lazy and Haven't written my Speculation of evidence towards one of the fighters, but I'll say something about the Bonus thing
I'm starting to lean towards it being Some sort of Echo for one of the Last 2 Fighters, instead of an Individual Bonus Fighter or any sort of Bonus Pass or something, Here's why
-An Echo of one of the FP2 Fighters would sense as to why the Bonus is only for CP2. I would believe that if the Bonus was a Full on Individual Fighter, they wouldn't make it free just for purchasing FP2. they might either have discount at that stage, or make it free if you Bought BOTH FP1 and FP2, but this bonus thing seems to be tied only to FP2
-Sakurai has stated only 2 Fighters and only 2 opportunities for the Presentations. I'll take him at his word when he says this. This would mean that this Bonus Fighter would have to be tied to one of the Last 2 Fighter Presentations, and not be separate at all.
-Technically, Sakurai never said no echoes for FP2, like he did for FP1
As for why they would even have an Echo though in the first place for one of the FP2 Fighters, It's likely that the character is as important to the franchise as one of the Last 2 Fighters that's being represented, so having them included in Smash is also a priority. However they might have felt that fitting in their personality or quirks through just an alt was not enough to represent them. This would especially be true if the Character was from a 3rd party, as the 3rd party has to give their piece into the development of the characters in Smash
The Reason Hero, Byleth, and Steve/Alex/Zombie/Enderman work is because they are mostly interchangeable, whether it be in Personality, Abilities, etc. Zombie and Enderman are kind of a stretch, but there were alts the Player character could use in Minecraft that matched the Zombie and Enderman alts. it seems in the case of our Bonus Fighter, they aren't interchangeable, or at the very least, the 3rd party won't allow them to be