Is it possible that Richter was made first? Yes.
Is it likely? Absolutely not.
You say there's only low-res sprites of Simon's, but Super Castlevania IV's sprite has about as much detail as Rondo's Richter sprite.
You say they used Richter's animation as a base for Simon's, yet they clearly are using Simon's strut, attack and jumping animations etc. from the NES games as base, something that Richter most definitely doesn't have.
You say they based stuff off of Richter's moves in Rondo/SotN, when the amount of moves they've lifted off of Richter can be counted on one hand. They are also only using 3 of the original 5 NES CV subs, none of the numerous subs added in subsequent games like the Bible, Ricochet Gem, Bibuti, etc. that Richter had access to.
Richter has the same strut, jump and attack animations as Simon. The point is, there's no need to make Simon first to make Richter, there is reason to make Richter first to make Simon
And I suspect Simon borrows more moves than you can count on one hand: slide, dash, dash with spinning whip, tackle, uppercut, grand cross, down kick?, maybe more?. Regardless, there's more source material for Richter than for Simon, an Richter sprites already do everything Simon sprites due. Richter sprites are a superset of Simon sprites, hence the reason one might start by making Richter
Richter isn't using different subs because they are echos, I wouldn't expect them to.
You say they made Richter first and based Simon off of him, when it's much more likely that they looked at Castlevania's history to see what they can draw from, Simon is an amalgamation of all Belmont characters, it's just that he draws more from Richter than others because Richter has the most mobility options compared to other Belmonts. They could have just as easily given Simon just the generic CV slidekick without the second part that lifts one off the air, and given him an uppercut animation based on Julius or Jonathan. But they decided to go with more of Richter, because the Belmonts all are very similar to each other, Richter is one of the most popular ones, and they already had the Airslash, Uppercut and Slidekick animations.
Is Smash Richter taking anything from other Belmonts? Is Simon?
This paragraph is basically what I'm saying, most of what Simon is doing is taken from Richter, hence, why not make Richter. I suspect the only reason Richter is even in the game as an echo is because it made sense to model the animations using him first. Richter isn't that popular. Sakurai made a point to mention that Simon's moves in smash are based on Richter. They could've given Simon the generic CV kick, but they didn't. They made Richter and called him Simon
This isn't the first time that a Smash character draws from other characters in their respective game or series, either.
Remember that Ness never used PK Fire, Thunder, Starstorm or PSI Magnet in Earthbound - those powers were utilized by Paula and Poo. The only special that Ness used originally in Earthbound was PK Flash, which he received in his second iteration. Nobody would ever claim that Sakurai wanted to make Paula first and decided that Ness should be the character instead.
Which is fine. But Ness's animations aren't sourced from the other characters (as far as I'm aware). All I'm saying is that as an animator, when translating these moves to 3D, the sensible way to do it would be to start with a model that looks like Richter so you can line the source material up (considering Richter will be playable anyway)
And I'm not saying they made Richter fist, just pointing out that it's likely. The reality is likely that they worked on both simultaneously, Richter simply may have been completed first. I would just assume those specific moves were first animated with Richter. We have a leak that guessed Richter when noone else did. The leak is based on demo code which would be incomplete. The leak only shows Richter. Just pointing out it's very possible that a specific version of the demo would only have Richter
If it ends up the leak is fake, no big deal. But Simon's absence isn't enough to discredit it. Even if Simon was first, they may still have been tweaking his model