OP, you've asked a question and people have told you the reason why, yet you take personal offence. Most teens do not just have bad mental health, yes some are more susceptible to MH issues, but there is likely a trigger/cause. Maybe you have rightfully chosen not to discuss as that's not thw point of your post. However, there may be something your DC is yet to disclose- bullying, witnessing a traumatic event, etc.
You are understandably overwhelmed and at he end of your tether due to what you've explained in your posts.
The SW has to speak to both DC individually, it's their job. Please try to see it as your child's best interest in protecting them, rather than an attack on you. Whether or not your DC1 has complex needs is irrelevant, they will document that your child is limited or unable to express themselves.
Imagine a situation where a child has significantly MH issues due to abusive parents. Parents lie about the current situation etc. That child may/may not disclose abuse. Later, child is significantly harmed or killed.
Serious case review highlights that no one asked the other children how they were feeling or what happens in the household. That one contact by themselves may have been the one chance to help those children.
In cases where parents/carers are manipulative, if more than 1 DC says similar things are happening, then that helps to prove that the child/ren aren't lying.