Seriously, I would be very surprised if Nino is not the kisser. Everything tends to that seriously (and I'm not even a Ninofag, I belong the Miku church). The earring chapter, now she calls the MC Fuutarou (or Fuu-Kun) like the bride, she is full attack mode in love, so she could have the balls to kiss him.
And with the Itsuki chapter today, Negi shows us than he can just write what was obvious (it was the case, Itsuki being Rena was so obvious before).
If the kisser is the bride (not 100% sure, but very plausible), we have the end game.
The only gray area is Yotsuba imo)