Leave it to Makoto to have a summer vacation that doesn't really line up with what we would think of as a typical vacation. He spends it training and researching. One thing I am sort of sad to see get cut is the time he spends with his students in Rotsgard during this time. They are also diligently training during this period (even though the Rembrandt sisters need to go home halfway through).
It has been a while since I read the WN, but I don't remember this mana matter stuff. I remember the book that the librarian provides had knowledge in it about the nature of magical incantations, not the physical manifestation of mana. So, this might be a part of the story that changed as it evolved from WN to LN. I am interested to see where this thread of the story goes.
I had complained in previous episodes that Aqua and Eris's humor was not really coming through as much in the anime adaptation, but they certainly did this episode. The whole sequence around the ice barrier was great. More silly forest ogres please! Also as a side note, this sequence happened (metaphorically) off screen in the WN version, so it was nice to see it animated!
This episode actually sets up a lot of things that will be important later in the story. It was awesome to see Shiki get to flex a bit as well as Mio show off her latest toy. In the end, we didn't get to see Tomoe do her thing, but a true samurai never flaunts their power...or something like that. She was probably more interested in eating persimmons.