The Idea was more along the lines of they telling the 3-A about the Omiai as if it was done, and the Headmaster about the possibility of one (As in, give me a bit of time to decide gramps...). Nothing concrete until the Headmaster loses his patience at their slowness and seal the deal without their consent.
As for the pactio, IIRC, they're a combination of potential (of the minister) and necessity (of the magister). Negi din't needed a Fighter as much as he needed a Healer when he pactio-ed with Konoka, and by not having a lot of ministras, he wouldn't get Mille Vinculla. Making another artifact is not that difficult.
Lets see: Negi is a natural protector, a book worm with a serious trauma and motivation, and Konoka would naturally need him at the front-lines. Either he gets a Shield+Sword Combo to function as a Warrior Mage, or a support set of Book, Cape and Light Armor, to work as a combat mage. I'd pick the second so he later works in tandem with Setchan.
The rest is a bit of research (most of the artifacts use the magic language of the magister - I'm guessing Japanese for Konoka, unless Negi is the one teaching her magic).
As for healer, well there's another one available: Ako.