I am not sure the malathion caused your leaves to turn yellow and drop. We spray with malathion here (mixed fairly strong) and have not had that problem. I am thinking that maybe your dendrobium is one that looses it leaves in the winter? Or it could have been the bugs that you were spraying to kill. They may have damaged the plant so much that the leaves fell off.
The black part of the cane could be caused by sun burn as well as a bug.
If the canes are still green and there are plenty of good roots, I would not cut the orchid up and attempt to clone it. It will probably recover in time on its own. If there are no viable roots, then you can consider attempting a clone. This is a time intensive process though. It *might* be easier to simply repurchase the orchid and start anew.