Shadow Knives is a great spell and I've love to learn it. However, I also feel like leaving the College behind will be an opportunity to stop just filling out our spellbook and instead focus on other lines of development. We have a ton of options for that; coming up with Fog of War spells, refining our enchanting skills, bonding a familiar and building up our bond with them to take advantage of some of the
really shiny familiar power options, creating an equivalent of the +1 Magic staffs that apparently they used to hand out, making
variants of the spells we already know, and more.
Basically, we have a wealth of cool things to study. "Can I kill a heavily armored opponent within short range" is the problem that Shadow Knives solves, and it's a problem that sometimes needs solving, but it's not one that Mathilde is going to be lacking alternate options for in this campaign what with all the mercenaries and dwarven heroes running around and her spell list already containing multiple options for dealing with such opponents, most notably Bewilder. Thus, it isn't worth the cost to buy the opportunity to learn Shadow Knives, or indeed any other spell, for this trip.
We'll be back in the Empire and able to learn them all eventually.