I don't know if it is a bug or not but the synthesizer recipe for sunflowers is the same as the one for dandelions. Will sunflowers work as well as dandelions?
Planting the saplings makes the air at the bottom of the mine breathable.I fill my hotbar completely before starting to mine so that I don't have to worry about the nasty things effecting me. Out of curiosity, how do you mine if you can't look at what you are mining?
I made a jetpack and made short mining trips to those catacombs that are all over the place since they are generally a gigantic death trap. As a result, I never really had to worry *too* much about air quality as I'd just fly back up when it started to become a problem. I also didn't have problems losing sanity when mining soul sand, netherack, etc so I'm not sure what else could be draining your sanity beyond just being below a certain depth where it slowly ticks down.
Once I had looted/mined enough materials, I built a digital miner to retrieve stuff from below. That task was made easier by the fact that you can get some of the necessary parts from the second shuttle crash site instead of having to craft them.