If Akamatsu didn't plan to reveal who Negi ended up with, why did he build up the last chapters like there would be some grand confession or revelation and then ignored the issue altogether in the final chapter? Not cool.
I guess its okay to end the series without revealing Negi's partner in itself and make it an open ending, but obviously this is NOT how to go about it.
3) About who Negi married I think it was Asuna after all, based on the scene at the end of the last chapter where we see Asuna and Ayaka with Fate and Negi in the background . That is clearly a wedding scene, based on the background and the dresses worn. Now consider that and these following excepts from Ayaka's the wedding scene with Asuna/Ayaka and the english translation:
After the establishment of the Issda, she continued to stay by the side of Negi and Asuna and became a major source of support. To me, the wording "Stay by the side of Negi and Asuna" emphasizes a relationship NegiXAsuna. Also, a supporting role seems an appropriate expression, when talking about a couple.
Followed by this excerpt, also about Ayaka:
She is happy simply standing to one side behind Negi Okay, so I don't know japanese, but if the english translations above are correct and unambiguous, it doesnt seem Ayaka is it together with Negi - because then you would say something simpler like "She is happy standing beside Negi". To say "To one side behind Negi" implies an supporting role.
So we have a wedding scene, two couples (or maybe Fate is just a best man?) and Ayaka is said to be supporting Negi and Asuna.
I had dismissed Asuna as Negi's partner based on her reaction earlier (when Negi whispered to her who he likes). Now that this final chapter is out, I suspect that was all just a ploy from Akamatsu, without any real meaning (or rather held no meaning in the story, but to entice the readers). So my vote is for Asuna!
EDIT: As someone pointed out, if you compare page 16 and 17, you see Asuna and Ayaka are attending Natsumi's wedding! Maybe NegiXMakie then.