The thing about VBS Rin is that it'd be too fast for her to get another 4* in Kohane5, unless they randomly do 3OC and give her the 3* there, but I think it's likely 2OC + VS, especially if Miku isn't at the end of June, so Rin is pretty much either Kohane5 or end of July (if they do Mizutoya mixed?)

If Luka sings for Niigo (Mizuki5?), Len for Ichika5, MEIKO for Airi5, and let's say KAITO for Tsukasa5 (could parallel 88 with that, especially if they do the funny 88th event song thing), that leaves Rin and Miku for VBS, assuming they don't repeat VS or have a singing VS that doesn't match one of the VS in the lineup, which hasn't happened at all since Airi4, I believe?
I can personally see now them delaying Miku yet another event so she gets a 4* in Kohane5, since she has barely been in event songs this uh... last year? (Although she makes up for that with the new unit songs and all the covers she is in, I suppose?)

Also I believe there's a chance they actually do three events in September (WxS lims, Niigo, and L/n). Since they skipped one mixed already, they may skip another, to "make up" for last year's mixed-only month. But if not, then I agree Niigo Luka could just be in a perm mixed (or L/n Len, if they actually leave L/n rather than Niigo for after anni)

By the way, speaking of Kohane5 and Miku earlier: I've been thinking about the case where they don't do Miku lim to put her in Kohane5, and I agree with you after all that it'd likely be a 3OC lineup.
However, as outrageous as it sounds, I think there's a fair chance it has Airi. I know it'd be her 5th lim 4* in a row, counting WL and Sanrio lol, but I feel she's a perfect fit in there. They usually have at least one character in lim banners that is most due for a lim, and she is among those right now. Otherwise the lineup would pretty much consist of characters that have had lims since NY.

It could be like: FES Kanaena + Emu, Honami/Ichika, Airi
Could do Akito over Airi if they go with Ichika, since I believe repeating Akito and Honami lims together so fast may be too much, but would they do a Ichikana banner yet again? For the fourth time in a row for Ichika? I like them but... we need variety

Also laying out all my predictions:
End of June Mixed - FES Kanaena + Emu, Ichika/Honami, Airi/Akito OR Emu, Akito, VBS Miku
VBS - Kohane, Akito, Miku [Miku sings] OR Kohane, ???, ??? [Rin sings]
L/n WL

End of July Mixed - Toya, Mizuki, Niigo MEIKO (Rin shop)
MMJ - Airi, Minori, Shizuku? MEIKO shop
VS WL

WxS lims - Tsukasa, Nene, Rui KAITO shop?
L/n - Ichika, ???, Len [Len sings]
Niigo - Mizuki, ???, Luka [Luka sings]

(wrote a lot as always. sorry)