Just don't tell them about it. Come up with an (unverifiable) excuse for the scars in case they notice them and ask, and they'll have no way to know for sure that it's self harm. It would be discrimination for the to exclude you from military service because you might have a history of self harm if you really did run through a glass door as a child, so I doubt they'd exclude you for it. For the joint hypermotility, I don't know exactly what that is, but it sounds like the main symptom is joint pain, in which case you simply don't tell them it hurts.
It's probably illegal to lie on military service eligability stuff, but I get the impression that people probably lie about minor stuff like that all the time, and if they find out then probably all that happens is they don't let you in (or kick you out, if they find out later), which is the same as would happen if you told them the truth about it from the start.
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