Tapping into one's shadow power did not drain one's light and increase the amount of shadow one had at one's disposal.
Shadow hands were energy and could be affected by the passage of time.
Shadow was potentially not always connected with evil, possibly only becoming evil when the Brotherhood of Makuta betrayed Mata Nui. It was likely always suspect, but possibly not always shunned.
There couldn't be Toa of Shadow because shadow was considered an evil element in the Matoran universe.
Shadow was not always an evil element, but came to be associated with evil because of what the Makuta did. There were no Toa of Shadows because the entire Brotherhood of Makuta had that power and had members stationed all over the universe.