Not necessarily. For something to be considered a disability, it has to actually inhibit your function in some way. "weak bones" is not a medical condition, perhaps you are referring to osteopenia or osteoporosis, which does not in and of itself limit function unless something happens, like a fracture.
I would be . Curious to know why you use the word "abnormal". People have arches that flatten. The word flexible flatfoot or hyper pronation is often used. Orthotics from a conservative standpoint can help. Disability in terms of what? Getting out of the army? Collecting social security? Or future foot problems?
Yes. This could lead to fractures and injuries that may cause future disability.
Why are they weak? Abnormal weak bones? Fragile arch? What exactly is meant by this? Are they so fragile that they break? Or are you talking about a flatfoot type of problem? There is a condition "brittle bone disease" this is a problem because bones easily fracture. Are you talking about something like that? A disability would imply you can't do certain functions is this the case?