Ultimately they have 20 minutes to get a story out so they had to take shortcuts on certain factors – such as complex ways to trick ponies into thinking it’s not a changeling. That’s the real reason. But for the sake of argument they could just trust Twilight enough to go with her judgement.
Oh, right, right— straight up dropped his ass to the asphalt before he even got to the throne room.
I think the Crystal Ponies are just hella more suspicious of… basically anything that moves suddenly or makes a loud noise than regular Equestrians. They’re like a nation of Fluttershys.
It is kinda odd that Twilight would be that quick to hop on the Crystal Hooves Train given that her suspicions were the only thing that saved the day the last time Changelings were hanging around Princess Lovebutt.
To be fair in Twilight’s defense, she doesn’t live in the Crystal Empire so she wouldn’t think that’s Thorax’s disguise in the first place. I feel like that caption would be better suited for Cadance since she does live here.
You’re on the right track here, but the dropped ball was that she trusted Spike… to, y’know, be Spike. If that’s really him, she’d have no reason not to trust his choice in Crystal Pony friends, but it was kinda doofy to take it for granted that he wasn’t a Changeling leading in a co-conspirator.
Although… Spike seemed surprised that Thorax could turn into a Dragon and a rock, so it’s possible that Ponydom in general is unaware that Changelings can turn into non-Pony things, and she assumed he was a kind of mimic-proof failsafe.
Nopony trusted Twilight or Starlight. They even temporarily distrusted Spike the Brave and Glorious. It doesn’t matter that he was an apparent pen pal to Spike, they still should’ve been at least a BIT suspicious of Crystal Hoof, because it’s been established changelings can shape shift into family and friends to feed off of your love.
Except as far as we know the only way they have to detect changelings is though knowing the pony and asking them something the Changeling wouldn’t know. She trusts Spike so likely thought that as Spike was his Pen Pal he would have done something to already check. It might be a weakness but there was nothing that she could have done to determine if he was one or not and it would be seriously OOC for Twilight to tell him to leave.
At this point in the episode I kinda thought Spike would be like, "Twilight, I trust you – this is my changeling friend, we really need for help getting him to be accepted, what with you being the princess of friendship and all."
But then that’s kinda what she did after the song, I guess.