Craft magic would be one way to do it, but there are others, such as MuTe to turn silver liquid or CrIg to actually melt it inside a mold. Which one is easier? That depends on your maga's specialties. She might even decide to reinvent her mirror-crafting spell to use T:Group directly a few years down the road.
I think it is a bit more complex than using a smartphone, since she is casting the spell at the same time as she is aiming the mirror. I'm even wondering whether using Per + Finesse is the correct combination, since it is actually a physical manipulation of the mirror to have it catch the correct frame. But you are correct that this is essentially a minor detail, unless there is a time-senstive lement at work. So I'd also be fine getting rid of the roll altogether, but stating that one may be required in exceptional circumstances (the modern equivalent would be getting the perfect video of the winning goal in a footbal game).
Again, most of the time the Finesse roll won't be an issue, as this mostly happens "off-camera". But shaping and polishing a silver mirror is a relatively time-consuming task for a skilled craftsman. She needs them to be circular enough to serve for a T:Ring spell, as well as smooth enough not to distort the images. All of which is why the Ease factor should reflect that, so I'd stick with a 12 for a useable mirror. With her current Finesse score, she can easiely make several dozens acceptable ones and a handful of higher-quality ones in an afternoon.
Unless you plan on having her make and keep several hundred of her pictures every year, the cost shouldn't be too much of an issue either. I'm assuming she will offer some of them to other magi, so they would certainly be ready to pay in silver. They would certainly be interesting in Jerbiton circles, at a minimum.
It's not like this saga will be about counting pennies and pounds of silver.
The good thing about the narrative approach to post-Gauntlet advancement is that you get to tell us what happens to her after she becomes a maga. The actual xp is based on her actions and decisions.
Rafael linked to an old post about the advancement guidelines. An updated version (not much changed) is available in the About the Ars Magica 5th Edition: The Heir of Laimunt Valley category - #3 by Arthur post.
The main thing to realize is that the guidelines represent an average over time. So if she spend a year reading good books, she might gain 60 xp in that year. The following year she might spend time travelling around and socializing, only gaining 30 xp.
Where that xp is spent depends on the narrative. And still need to respect the rules (single source of xp in a season). Note that I've often seen a short story (5 xp) done during a season of lab work use some of the story xp to reflect the exposure xp. So, for example, 2 xp would be gained in Magic Theory (lab work) and 3 xp in Area Lore (events of the story).
She could still do it using a level 2 spell, it simply wouldn't be reliable (5 + stress die means she would succeed more than half the time). If it means she can avoid the risk of botch for casting a fatiguing spontaneous spell, it might be worth the extra time.