Are your other orchids the same as the largest one that is thriving? Sounds like the mix you use has been tested and you have a happy plant as a result then I'd stick to that.
I have no experience with coconut fiber. I have used the chunks without success, roots stayed wet for too long for me.
As far as how far to cover roots, I keep covered what was already covered. If there are air roots then I keep them out of the mix.