Kouhaku? hmm i dont know, seems far from their league, but who knows, with the downfall of music sales from the past decade, they might invite artists who are 'moderately succefull' nowadays (who would be called a flop years back).
they invited AAA and HY, they dont have that amazing of sales, but they did get invited... for BENI i dont think theres a chance actually, she doesnt have a song which was a popular hit just her album, while Che'Nelle had a hit with her cover album and now the song Believe are a extremelly success hit, but her sales arent that outstanding.
we shall wait and see...
but Best Hit Kayousai shall announce artists line-up soon, as the show is in November, maybe mid-October we'll know. BENI will surely make to FNS Kayousai, because she was invited to FNS Natsu and this will grant her a position for the usual regular show in begining of December
HY might not be selling that much anymore but they used to be HUGE. One of the best-selling independent bands for a while. 366hi was a major hit. It's just Whistle that did moderately and Parade that did meh... their status is still high, and sometimes for Kokuhaku it's more overall status than current popularity that counts.
As for AAA... I'm 100% sure that's Avex's lobbying, to be honest. They're only doing OK and there are tons of other more popular and more "artisty" artists available over them.
As for BENI appearing at Kouhaku... I believe that they'd either do Che'Nelle or BENI, but not both, in which case I think Che'Nelle might "win" because she's been selling more and because she has the 'novelty' factor, I guess : P