Probably didn't want to take away from the actual SSJ form similar to why Vegetto showed up in the manga before Gogeta instead of it being Gogeta in both :troll
I think Janemba's sword had more special characteristics since it was magically created and Janemba could launch Ki blades from it to attack from a range
Does Fat Janemba need to be that high? He didn't damage SSJ2 Goku that badly, and I'm assuming SSJ2 Goku is 25 on that list going by the 4x SSJ3 multiplier.
Considering Kuririn's Kienzan cut Freeza's tail off and was dangerous enough for him to need to dodge it repeatedly, plus Future Trunks' Ki sword (even before he powered it up with the Genkidama) could cut into Merged Zamasu's skin, I think the sword definitely makes Janemba much more dangerous.
It's hard to say since he only ever used it against Goku, but given that SSJ2 Vegeta was able to destroy it, I doubt it'd work on beings a lot stronger than himself. Unless they were stupid and just let themselves get cut by it. But since cutting moves seem a lot more effective than melee or...
Cell stated he can wipe out the solar system with his Ki, and that's his SPC form. All there is to it. If no one stronger than he showed up over a period of centuries, then yeah he could destroy the universe over time. Same with Kid boo.
Unless you abide by that retarded logic of Janemba > Hildegarn because the latter was even with SSJ3 Goku despite how obviously haxed he was in that movie, I'd say Gotenks wins. I don't think Janemba did a huge power up at the end.