There seems to be a real lack of information about the reality of their relationship
Absolutely, but again, this conversation isn't just about this relationship. We're talking about what appropriate laws might be, and you don't make laws for each individual human.
The fact is that this particular relationship could very well be healthy and mutually beneficial, but we don't know. Similarly, an individual might be really able to process alcohol well and drive absolutely safely well over the legal limit. The way we work risk management into law is not by legislating around idiosyncrasies, but by drawing clear lines.
Documentation of this claim?
Documentation that breath play exists and that people are into it? Do a simple google search.
What real risks did she take on? IT seems to be a bit of bruising.
That was the injury that she took on, which is not the extent of the risk.
She was supported by clamps from her nipples such that she had to stand on her tiptoes. I don't know how strong those clamps were, and neither to you. If she had gotten spooked by a door suddenly opening, or exhausted if he got distracted and left, or slipped if the floor had an overwaxed spot, it could have ripped her nipple off. It absolutely depended on his skill, knowledge and concentration to keep that from happening. Outside of sex, we don't allow minors to take on that level of risk.
Again, these are just the activities we know they engaged in. Other activities carry other risks.
Over and above that, there are real emotional risks. I didn't want to bring them up because they are incredibly tricky to define, to track, and to prove. But I think being ordered to write this statement and post it online:
"I am a slave, a filthy dirty whore and want to be treated like that, I want to be exploited, harassed and thoroughly humiliated until I can barely hold back the tears and silent cries who suffocated inside "
While it may be an excitingly lovely dirty fantasy fulfillment for some, is also pushing a lot of buttons in a teenage mind that biologically processes both risk and emotion differently from an adult brain.
Again, Sweden has a law preventing inflicting pain and humiliation on a minor. The question at hand is whether sex is a valid exception.