If you used drano for a hair clog in shower drain and it didn’t work, can you use this after? Would there be an adverse chemical reaction?

I didn't use it immediately after Drano, but I used it after trying Drano for months if not years with limited results. This worked the first time I used it and three months later the drain is still flowing free. I was frankly amazed. I've never seen a product work as well. (I did flush with boiling water after letting this work overnight. I don't know if it enhanced the effect, but I am extremely pleased with it.)

Can it be used on pvc pipes?

At least in my bathroom sink, it was a disaster. This product dissolved something and the pvc pipe detached from the pipe under the sink causing a leak that damaged everything I had inside the cabinet and the hair was still in there. I had a clogged sink and now I have a useless sink because I have to find a plumber to fix the pipes. Thankfully I have two more bathrooms with sinks that I can use. Take the risk if you want, I'm not using it never again.

No instructions on using with garbage disposal? Do you do it the same way as if you just had a drain with no disposal?

My disposal was clogged up when it quit working and the water was also backed up in my dishwasher. I ordered some more Green Gobbler with the 2 spouts and used one in my disposal, waited 15 minutes +, then poured a pan of hot water in it again about 15 or so minutes. It just sat there and didn't go down, but the next morning it was down. I poured another pan of hot water in and the clog was gone. Had to dip out my dishwasher, but then everything worked well. BTW, I wouldn't recommend standing over the sink after pouring in the hot water, because it does fume some.

Will it work on clog that is several feet past the trap in shower?

I think it depends on how long a pipe you have and the pitch of the pipe. I have a 30 ft almost level drain from my kitchen sink. I had a very slow drain but it was many feet past the trap so the blockage did not backup the water quickly. I tried 4 applications of Green Gobbler over 2 days (yes I used hot water, 30 minute soak, overnight soak, boiling water, etc) and it did not work. I will try the Green main line stuff, and the bio-clean. Hope something works.