There's a chance it might not be leakage at all, if the cement board goes all the way to the bottom of the shwer pan (in other words, it rests on the membrane) and there's no slope under the membrane. If that's the case, water could be wicking up the cement board until it reaches the top of the pan liner, and then spilling out, APPEARING to be a leak. Still just as bad, though. Lets start with the first question-- is he laying the membrane down flat on the subfloor, and is the cement board then sitting right on the pan membrane, or is it up an inch or so off the bottom?

Sounds like they didn't preslope the pan, and it may be holding water now, which would be why you can't get rid of the mildew. Not the fault of the guy who installed the shower-- presloping the pan membrane became code 12 years ago, and there are many-- even inspectors-- who STILL don't know it's now code. But that's most likely the reason you're having problems. Only way I know of to rectify the situation is the tear it out and put the pan together right.

You should be able to shower with the pan liner in, cement board up(actual cement board), and shower curtain. Build curb, floor mud preslope, install liner(no fasteners), and another 1“ minimum floor mud on top, float curb. Cement board walls, silicon the edges, tape the seams, thinset level, and red guard the curb and walls. There won’t be any leaks at this stage. The tile provides merely aesthetics in the grand scheme of it all. They help shed water but offer very little to the game of waterproofing. It’s an illusion! Red guard is completely unnecessary and I don’t recommend it on every surface. Don’t use it on the bottom 6” or liner height of wall, or the top 3ft of wall, or on the pan floor, and never on drywall. The cement board will be submerged in the pan tile deep so it needs some breathing room. You’re just asking for moisture to accumulate in the wall but wait there’s a moisture barrier behind the cement board. That barrier got screwed up from the durarock install but you siliconed each stud, right? That’s called a hole in the plan. Welcome to the shower install conflict. For the love of grout don’t seal it! A 1/16th square. That’s impressive. The line of a dull carpenters pencil. You’d be lucky to buy board that’s within a 1/16”. I’ve worked with all kinds of framing types and they’d throw you off the deck if you were fussing over a 1/16”. Finish guys now that’s a different story. A 1/2“ square per 12ft, 1/4“ plumb average per 8ft, and a 1/16” level per 4ft in all reality is the best workmanship you‘re going to find in any home. Any thing else is an empty promise or a rubber square.

If that was directed to me, I wasn't hateful. I want to help people. I spent a lot of time responding to him on his first thread. And it wasn't as if I was an outlier telling him what he had needed to be completely removed and hire a competent contractor. For whatever reason he didn't heed that advice. Now It's leaking again, and of course nobody can tell him where that's leaking. We only know that the job was poorly done. We know that because it's leaking. We know that because it was patched. We know that from looking at the tile work and the curb. We all know pretty clearly that you can't "repair" these things (unless it's the shower door, for example). I'm serious when I say that I don't understand why he is posting here now - it's not hateful... we can't help him. Anything that anyone would say is a guess. We have no idea what's going on under the tile (other than it's not right). The most helpful thing we CAN do is reiterate for anyone else in this position is that you don't want to do what was done. I was a little miffed that he didn't identify himself upfront as having posted before about this project - could have saved some of the "Caulk isn't waterproofing" stuff that he's already heard. On a broader scale, I'm not happy with the situation in general - and that's not the OP's fault... but he's not really hearing what's being said either. The bigger situation is something that Sophie and Creative Tile have talked around the edges about: A custom tiled shower is expensive and requires a lot of skill and workmanship to do properly. Period. Full Stop. No wiggle room. No, "It's not in my budget". Homeowners are watching HGTV and have come to believe that things like curbless showers, custom tile showers, steam showers, multi-head showers, garden tubs, etc. are "normal" and baseline, rather than a luxury that requires a premium cost to do properly. I think the influence of HGTV has driven the cost of housing up tremendously. A lot of what is being "sold" is really trendy crap that will be dated by the next season - but the mortgage will go on for 30 years. There's a TON of really bad showers out there. House flippers in particular have done shoddy work installing these kinds of amenities that doesn't show up until the drywall behind the tile disintegrates several months later. I am really sorry the OP bought one of these houses that had shoddy as hell construction in the bathroom -- but the whole HGTV / flipped house thing is a scam and young home buyers, in particular, need to wise up. They are spending a lot of money on homes with a lot of flash and really poor construction. They will be defaulting on those loans if they aren't careful. But more to the point, the OP in this case was able to see why the original construction was crap, had good people telling him it couldn't be done on a low budget, we went into detail on the proper way to do things... and then the OP reverted to the doing things the wrong way and it didn't go well. I can't help him.