You're right, that should be more like...training with character "A" allows you to train with related character "B"
Rules of the Internet 45. Rule 45 is a lie. Check out my art if you notice.Last week's episode of Kai cleared up my memory of Cell's backstory better. Gero made Cell first, but the project was so long in formulating that he moved back to Androids to build something he could see in his own lifetime.
It doesn't explain why he built 17 and 18 after 16, given that Cell implied that Gero's computer was the one who came up with the idea that Cell could fuse with 17 and 18 to achieve his perfect form, so they weren't created just to feed him. Maybe Gero made them, like Cell, simply because he could, building a better human or something like that, and also trying to kill Goku in the bargain
16 was apparently intended as a Replacement Goldfish for his dead son. He was never supposed to be a fighter, and certainly not be fed to Cell.
19 and 16 were actually Androids...personality they do not have organically.
He could've brain-washed the siblings, but why do that?
Rules of the Internet 45. Rule 45 is a lie. Check out my art if you notice.... Which makes me wonder, why doesn't Cell have an in-built remote control function for the androids or, failing that, access to a backup copy of the remote?
Seems like a no brainer if Cell's first form is weaker than them.
himitsu keisatsu seifu chokuzoku kokka hoanbu na no da himitsu keisatsu yami ni magireru supai katsudou torishimariAgreed.
Well, also that the entire Cell Arc was constantly changing.
We all already know why, so no reason to dwell on it. At this point, we're trying to make up in universe reasons for it when there is no in universe reason. It's just that thing kept changing constantly.
One Strip! One Strip!

Actually, we stopped having unlockable characters when dumbass big name publishers figured out you could gate those behind a paywall instead of a skill/effort wall, which generated more money for them.
Back in the day, you had to beat the game in a specific way, input a cheat code or even do something completely counterintuitive to access the secret characters. These days, the secret character's portrait in the selection screen is plain as day... it's just that the publisher told the devs to stick that character behind a paywall so that they could make more money.
(And yes, this generally is an insistence of the publisher's, since game devs don't really make money from sales or DLC, the publisher does)
himitsu keisatsu seifu chokuzoku kokka hoanbu na no da himitsu keisatsu yami ni magireru supai katsudou torishimari