Juniper's alter ego doesn't exhibit any powers like the other young women who were also corrupted by magic. Juniper can't create mind-controlled slaves like Demon Shimmer, open dimensional portals like Midnight Sparkle, or manipulate plant life like Gaea Everfree. Juniper just becomes a giant, and not even that big of a giant. But from what we've seen, magic takes what the person desires and twists it to its extremes. Sunset wanted to be a princess and got turned into a demonic tyrant, Twilight (Sci-Twi) wanted to learn everything she could about magic and upon learning it came from another dimension (although this wasn't her fault), gained the power to force her way past the dimensional barriers, and Gloriosa wanted to protect her camp and make her campers happy, which led to her becoming Gaea Everfree and trapping them inside a dome of vines. Juniper, meanwhile, dreams of being a famous actress adored by her many fans. So when magic corrupted Juniper, it took her desires and turned her into a literally larger-than-life movie star who believes everyone loves her. Magic even takes it a step further and makes her alter ego more monstrous in appearance compared to her ideal self she saw in the mirror.
Starlight Glimmer's reaction to the Human world mirrors that of Princess Twilight's... except that Starlight is more shocked than panicked by her new form. This, however, has a logical explication. Sunset and Starlight's talk in Princess Twilight's library makes it obvious that Twilight told her about the other world. While what she told to Starlight about it exactly is unknown, Rainbow Rocks shows that she did told her friends about CHS, so is very likely Twilight told Starlight enough about the Human world to give Starlight an idea about what she would look like should she ever cross the portal.
Apparently at some point between films, Cadance replaced Cinch as Principal of Crystal Prep. Even without the whole "nearly destroyed the world" part, Cinch still blackmailed a student and outright admitted to knowing she'd been bullied and did nothing. Either of those things could lead to a school official being fired or forced to step down in real life.
Rainbow Dash and Twilight being the only girls to have their geodes during "Movie Magic" makes sense when you think about it. Rainbow Dash and Twilight's powers are by the far the most useful of the group. Rainbow Dash's super speed allows her to go anywhere in seconds while Twilight's lets her grab anything she wants without disturbing her studying and such. The other girls' powers are either too dangerous to have on all the time (Pinkie Pie, Applejack) or not useful to use at times (Fluttershy, Sunset).
Twilight being able to pick locks with her magic makes sense, as door locks are mechanisms of moving parts; with Twilight being as curious and smart as she is, she probably knows how the average door lock works and thus can use her telekinesis to pick them.
Sunset's Element has been all but explicitly stated to be the Element of Empathy. So how does Starlight get Juniper to willingly give up her magic? By empathizing with her.
It seems weird that Sunset would forget she has telekinesis when she used it during the first film after an even longer time away from Equestria. But during Camp Everfree, she tried to use it in her tent before learning that it was Human Twilight's special magic there. That idea might have still been on her mind - along with everything else she was worried about.
At the end of Mirror Magic, just before Juniper sets the Humane Seven free, one can see that Applejack is the one holding her friends on the one platform in the fractured mirror dimension. Normally this would be impossible for a high school age girl to do, even one as strong as Applejack, but this can be attributed to her special power being Super-Strength.
One unrelated to the magical happenings, but Sugarcoat can be seen doing the ballet pointe technique... on what seem to be regular sneakers, not proper pointe shoes. Since even properly done pointe work (as in, with proper shoes, and toe spacers, and pads, and so forth) can still lead to borderline Body Horror effects on a dancer's feet, one better hopes she doesn't do this frequently. Although for what is worth, she doesn't stay on pointe for too long, and the "Dance Magic" music video practically doesn't use it.
Magic corrupts another young woman, but this time it's like the magic picked out Juniper specifically. It's possible that the magic coming out of the portal is seeking out individuals in trouble and turning them into monsters. In the EG universe, just having a bad day can lead to you becoming an insane monster if the magic of the portal is searching for troubled people specifically.
Tying to this, Cinch got replaced by Cadance; seeing the sort of stuff she was doing, it's extremely likely she was fired, and her reputation is probably in shambles. Now, there's magic on the loose that might be targeting troubled individuals and turning them into monsters/giving them powers/etc. Do the math.
A.K. Yearling is implied to be Daring Do in this universe too. That means that all her enemies, both mundane and supernatural, could possibly exist in the human world as well.
Twilight is happy with her new friends and conquered her demons but the viewers know it's only a matter of time before the girls find the source of the leaking magic and learn it was Midnight's fault that more people were corrupted by magic.
The novelization amps up the fridge horror since it's explicitly stated that the magic is getting inside a bunch of mundane objects (mostly shiny things with reflective surfaces, like other mirrors or countertops) in public spaces. It's like Ascended Fridge Horror.
Juniper Montage's entire situation after "Mirror Magic" is quite bleak. As noted in her character page under How the Mighty Have Fallen, Juniper had a potentially bright career in the movie industry, what with her uncle being a famous film director and her being his gofer/stagehand for what is implied to be most of her formative years (all the films he has made in the studio "Movie Magic" takes place), coupled with her supposedly vast knowledge about movies and filmmaking and all the benefits her position gave her (such as being able to get close to movie stars and the like). She ends throwing everything into a flaming dumpster with her attempted sabotage of the movie, and thus gets fired by her uncle, barred from coming back to the studio for who-knows-how-long, and is taken away in disgrace. Then she gets a Burger Fool job at a local move theater's concession stand, in which she apparently doesn't last long (what with getting herself turned into a magically-enhanced monster and all — she's not seen or mentioned in "Canterlot Movie Club", which takes place sometime after "Mirror"). What's going to happen to her now? Add to this the fact that, as the niece of a famous film director, these incidents (or at least, the fact she tried to sabotage his uncle's big blockbuster film and had a short-lived stint as a Burger Fool) have a chance of becoming public and widely know, to add more salt to the wound.
