Chrom seems like a late addition too. It likely was just a "we don't have time to model out a new character, change the animation of this one move, get the voice actors for whatever character we choose and change the GFX to match." I don't blame them for not replacing Chrom at all.
I don't know. It seems like he clearly wasn't worked on much around his reveal time, but that doesn't mean he was a late addition. Echoes may have been decided upon very early this time. This is notable with Richter. He had to have been decided upon almost right away, due to how licensing works. It's likely that he got chosen not just because of the whole "moveset being similar", but trying to get the licensing for multiple Belmonts was difficult, so he went for one more instead, making the licensing easier.
If this is the case, it makes sense to decide the full roster just as early on. He may have chosen the Echoes around the same time besides Richter. All were fairly easy to make, and were popular. For all we know, Richter was the only one chosen somewhat later(around when they started worked on licensing out Simon and the Castlevania materials). It is possible Ken got licensed a little later, but it makes more sense he got chosen immediately. The other few Echoes were all 1st party and super easy to choose, especially since the ballot showed how popular they options were.
There's a good chance the full roster was chosen super early with no late additions. In Chrom's case, trying to remake the Final Smash was a low priority because it took too much time to make all new models. And again, it worked right for the purpose. Maybe at one point, he intended to, but realized it already fits.