I'm not sure Dai and Koakuma were ever intended to be characters, no more than the random fairies and ghosts and grimoires you shoot in the stages are characters. They have healthbars and serve as midbosses, but ultimately they were just mooks thrown in for stage progression. Later stage midbosses like Lily and Tewi were given much more focus (relatively), perhaps as a response to ZUN noticing how his previous midbosses were treated by the fans.
In regards to Satsuki Rin though, I think it's worth remembering that there are probably hundreds of discarded character ideas that ZUN never used, simply because they got tossed in the wastebasket long before they got anywhere. It's incredibly unlikely that ZUN goes with his very first idea for each character, so most likely there's at least 2-3 extremely rough ideas for each boss that he never followed through on. He's even mentioned these sometimes in interviews, like how Byakuren was originally conceived as being her more famous younger brother, and I think he also mentioned that he was considering an actual space alien to go with the Space Invaders theme? Multiply that by every single character in the series, and that's a rough estimate of how many "Satsuki Rins" have technically existed in ZUN's mind, however briefly, but we've never even seen the slightest trace of because there are no records. Apparently Rin got slightly further if she's mentioned in the code, but that's not necessarily significant. It's no surprise ZUN's forgot about her, since from his perspective she was just one of hundreds of bad ideas he never used.
