Im not a doctor so can't really comment on your condition. Your condition sounds rare, you probably need to consult an expert to talk about different weight training techniques.
On boxing, get down to a boxing gym and give it a go, as long as your arms aren't in constant pain or anything I don't see how it can be harmful to you. As you said, the movement of your arms, how you throw punches and defend etc., will be different to most people, but you won't know if this is an advantage or a disadvantage until you give it a go.