Meiling, Komachi, Reimu, Nitori, MND Iku, Chen, Alice, Remilia, Reisen, Mystia, Youmu, Suwako.
This looks decent. I can't say that replacing anyone would be outright better, but I can say that some of the characters are more replacable than others in an ideal party.
First off, Meiling is essentially irreplacable. Yes, I know Tenshi can tank pretty well too, but honestly speaking, Meiling's sort of in a tier of her own. She's pretty much undeniably the best at what she does. Tenshi has overkill DEF/MND and not much else, while Meiling has good enough DEF/MND but with much better HP and utility. And when affinities start mattering more than DEF/MND, Tenshi just sort of loses her charm (although she's still a fine tank for main game).
Reimu I would also venture to say is irreplacable. The only thing that can come close to filling her role would be if you have both Yukari and Rumia, and even then Rumia isn't even close to comparable until you're well into the Plus Disk. Well, technically I could argue that if you know what you're doing, Minoriko can be an acceptable replacement, but even then it's debatable. But at least you can make some sort of argument for Minoriko as sole healer in an ideal party for the main game, which you can't even do for Sanae or Eirin.
I'd personally consider Iku irreplacable. Now, I'm well-known for being a big fan of Iku due to my experiences, but she's simply the best at doing what she does: passing out offensive buffs. +72% ATK/MAG is simply staggering; nothing else even comes close. Ran is a good buffer because she can also buff DEF/MND and she can attack pretty well too; otherwise, in general and practically speaking Ran is just plain inferior at offensive buffing (and I can show you why mathematically if you want). Being the best dedicated MND tank in the game does help Iku's case as well (Patchouli can get better MND, but she actually has a reason for putting points in MAG while Iku kind of doesn't).
Lastly, I'd say that Mystia is irreplacable. Yes, Renko and Cirno can both do similar things, but Mystia has one major benefit that those two lack: she can actually do respectable damage. There will be boss fights where Renko and Cirno are more or less useless, but that will NEVER happen with Mystia.
Aside from that, everyone else in the party is pretty much replaceable. You could replace Youmu with Yuugi and Alice with Patchouli, and I daresay you won't notice much of a difference in the long run. One could make the argument that Nitori is irreplacable, and indeed she is the best at what she does, just like the other characters that I called irreplacable. However, there is one difference: she is only one damage dealer out of at least half a dozen, while the other characters that I called irreplacable are oftentimes the only characters on your team filling that role. Thus, trading out one damage dealer for a slightly worse one (like replacing Nitori with Shikieiki) won't affect your team as much relative to trading out, say, Meiling for Tenshi. However, if I were to make an ideal team, Nitori would definitely be in.
