Shower Waterproofing

Just joined the Isaac Cult. First time tiler here. Looking forward to some solid advice & feedback from y'all!

I hired a general contractor to install cement backerboard (that I'd already purchased) in my bathtub surround so that I can tile it. He did a couple things that were very inconsistent with what I've been learning from folks on Youtube, so I halted the process until I can get some things clarified here... He's a nice kid and does excellent work on other things, so am hoping to resolve this ASAP.

1) Instead of using thinset mortar to join the backerboard to the drywall and even out the wall inconsistencies, he used Silverset drywall joint compound. He's going to put on 2 more layers. He claims that a couple layers of Aquadefense will waterproof it. However, I've heard that drywall mud can break down over time. Should he have used mortar instead? If so, am I now doomed to try to scrape all this off?

2) As the tub was tight against 2 of the 3 walls, the backerboard, which was installed on top of the tub lip, juts about 3/8 inch out from the tub lip, which is of course only about 1/8 inch thick. My guy filled in these gaps with joint compound, (again instead of mortar, which is what everyone else uses) and says we can waterproof it with Aquadefense so it'll be okay. I can't see how that would be a good thing, having joint compound right against the tub edge, waterproofing or not. Am I being paranoid?

3) When he initially hung the backerboard, rather than using the fine fabric-type tape at the seams, he used standard mesh, and then had me put a first coat of Aquadefense over it. There were some sizable gaps between some of the joints, but with enough of the product I was able to seal them. Do you think this will hold out? Will there be any tiling issues with the un-smooth surfaces?

4) Instead of using the backerboard I had purchased for the wall containing the faucet, he went and bought a different kind of board. I suspect this was for ease of cutting without having a circular blade. Now that I look at it, it seems it might have a coating that might be paper... Does this look familiar to you guys? Is it acceptable for use in a wet area?

Thanks for any qualified advice or links you could give!

Eric