It actually takes over a year to add a character, but regardless, I doubt that an Echo takes up 1/6 of the time.
Firstly, looking at Melee's clones, we got six with the remaining time that the game had, and in that case we know we couldn't have gotten more then one newcomer at best out of that situation as Sakurai
blatantly says they couldn't have substituted the six clones for a unique character:
Echoes have even less differences then the Melee clones had, so if six of the Melee clones didn't take up enough time to get one unique newcomer out of the situation, then six Echoes definitely wouldn't be able to contribute to one unique newcomer if they were added.
Even ignoring that, Lucina and Dark Pit (and Dr. Mario) were split off late in development, so evidentially they couldn't have taken up that much time and resources if they were able to do that and not hurt development in any meaningful way. On top of that, we know their creation process is much less demanding as they're only balanced against their counterpart instead of every other character and don't have too many core differences.
In short, nothing about your scenario would be likely to happen if that one Echo character was made. If that one was made, you still would've gotten the five unique DLC newcomers at the same time you would've otherwise, they don't take up nearly enough time to push development around in a meaningful way.