I also speculated Buono members. There's a rumor in Thai also about Momoko will not come. Now Yurina is confirmed to come too.
BTW Berryz had hold a concert in Thailand so all 7 have been in Thailand.
I don't think Buono! is active these days but I know that Chii could be busy with that Sugar Spot stage play. Maybe the other girl who stays home is Momochi.