Yes the place I go to said they use Consan 20 but they need a license to use it. I think that only applies if the plants are to be sold. Same thing with other chemicals if you read the fine print. No idea why.
I have heard that rainwater's the best but realistically whatever bucket or however you collect it in will have dust or residue in it. Might not matter though. If you do mist your plants I'd say rainwater.
I can't say that 100% of those orchids on eBay from China weren't propagated in someone's backyard. I mean if the conditions were perfect they could be divisions from a garden. Not sure if that's possible with Hangianums. The climate and everything would have to be just right to...
Vitamin B1 and liquid seaweed are both supposed to be good for orchids. Also pouring through a coffee filter may help. But yes I assume anything can build up in the pot. If you have some moss on the top you could just pick it off and replace it a couple times a year.
I mean I've gotten some unhealthy plants on ebay but there's always a risk no matter who you buy from. Like after you buy from a good mom and pop shop your plant could still die. It might not like the environment or repotting or whatever.