Blob,
I would venture that the mixed laterality and the SPD, hypotonia, motor and coordination issues, and processing problems probably all arise from underlying differences in neurological development, and that the mixed laterality is a result, not a cause.
As for his issues with midline crossing, etc. not being resolved in three years of weekly OT, what methodologies and approaches has his OT been using? What kind of home program has the OT been having you do? Has he been assessed for retained infantile reflexes?
Don't despair, and don't hesitate to switch therapists if your OT can't give you good reasons grounded in sound clinical evidence for why your child is not progressing. It took us four years of 3x/week OT and 2x/week PT using a multi-pronged approach with a team of specialized therapists along with a fairly intensive home program for my son to make functionally significant progress in these areas, although we could see changes in relevant measures on a fairly consistent basis during the whole process. He was just starting from a point where he has to make a lot of gains to see an improvement in his ability to actually do things. According to my son's therapists, swimming, horseback riding, and bicycling (if coordination allows at this point) are all very helpful in neurological repatterning, with the bonus that they are often not perceived as "therapy'.
