1) I use cement board screws to hang cement board. They have a much higher shear strength and they have self-cutting heads so the screws countersink themselves. Your screws will be under HB, so they won't see water and rust/corrode as drywall screws can do. But if I'm asked, and you did ask, yes, use cement board screws and not drywall screws.

They have them at the box stores. The ones I use are usually "Rock On" brand, though another brand can be used. They have a high-low threading on them and with the NextGen Durock they are very easy to install.

Honestly, you shouldn't have had to specify that the proper screw be used. Or that the boards be properly sized and cut.

2) Did it come with a rubber seal? They have a "disc" type of seal to go around the plastic box. Lemme see if I can find a reference...

Go to page 5 of this pdf, which is page 9 of the brochure. You can see the large rubber disc.

Without that disc, yes, you can fabric or mesh tape (if required) and thinset the gap. With a large gap, you might need a couple of applications of thinset, or if you have hot mud you can use that. A single large application of thinset could shrink/crack/sag as it cures.

3) Depends on the size of your tiles. If you're using a large tile, the tiles will get good bonding on the cement board and can easily bridge that 1" gap that you filled with thinset.

Now compare that to a mosaic tile. You woudn't want a mosaic tile to only be bonded to the thinset used to fill that gap. So for larger tile, you can tile right over the thinset-filled gap. For smaller tile, I'd recut the board so the tiles can better bond to the cement backer board and not just the thinset-fill.

I do have to say I'm sorry for your troubles. I thoroughly enjoy your posts and your projects, your eye for detail, and your perseverance as you've worked through all the roadblocks that have confronted you. It really is unfortunate when hired out jobs need redoing by DIY.

Thank you so much for your help!

I now have a clearer understanding of why tilers want to hang their own cement board. I didn't know about the small tile vs large tile needs regarding underlying support. Thanks for that.

Yes the disk I referred to as "non woven" facing is the one you called out in the PDF file and is to be placed around the rough-in. I will be carrying out that procedure as well as tiling the shower overall.

I will put alkaline resistant fiberglass mesh inside the holes and apply several layers of thinset in stages to prevent shrinkage. The tiles will be large 12x24, so I am glad it will not be an issue with laying over the thinset. I am assuming that I can do the same for the gap at the bottom. I had started working on the tile flange gap with the left over thinset from my floor install, but ran out. Then started wondering if I should just remove the whole panel and get a new one to better fit the tile flange and rough-ins.

If this bottom gap is a deal breaker let me know as this panel also has the wonky rough-in cut outs too.

This picture shows the size of the gap:

I have a box of screws (if I can find them in this mess :) that are for Durock. I might or might not change them out. I am going to call the drywall business and tell them of the drywall screws in the cement board issue. Maybe if I'm lucky they will come out and change them out.

They were asked not to put any nails in the fiberglass walling and I think they kept to this as I called them on it when they put the nail in. But I have found several on the cement board. I will take those out, if I can without too much trouble.

An actual employee of the drywall company (and not the subcontracted people who hung the wall) is the one that taped the seams for the fiberglass board. He used setting type joint compound as I had asked for and I am very happy with the results. It was a little hard to convince him that I would be happy with the results. He wanted to do the typical drying type compound, I could tell. He said the setting compound would not finish as well. I am very happy with the job he did. My paint job will add a slight roller texture anyway and I believe it will totally cover any smoothness differences between the two types of compound. I love my walls.

This might be my last tile job. I have one more floor to be done on the porch and I haven't decided if I should just put sheet product back on or tile the space. Tile would be very nice. And I think the floor would support a porcelain tile.

In your last paragraph you expressed sympathies for my troubles on this project. Some of my troubles are in my head :) I am starting to be reassured that these things will be ok. When I got my preferred carpenter on the job I was glad to hear that he thought the first carpenter did a job that was good, and easy to follow. My pocket door at the tile end of the shower was the most recent issue. I just gave it up after we re-enforced the studs with 1/4x3/4 steel straps along the sides, then glued and screwed every 6" 3/4 Sturd-I floor ply in between (sort of like an underlayment). Once the cement board went up it is very strong but there is a teeny tinny bit of flex. I am hoping that the door jamb, trim, tile, and thinset will take that out. As it is one would have to really ram against it to get it to flex. The actual cement board between fasteners flexes more than the door opening. I'm going with it.

ps, after editing this post a million times and having lunch I found my "Rock On" durock and wonderboard screws.

Enduring, we are at similar stages...and my DH cut one hole with less than a gap than yours, and he recut a new sheet. (He thinks he can probably reuse most of the first sheet elsewhere in the shower, though I think he would have redone it even if he couldn't). Like you we are also using 12 X 24 tiles, so what Mongo says makes sense, but I think the whole idea of trying to make this waterproof using the Hydroban, etc. makes us leery of trying to patch up especially right around the I-box where there's bound to be water.

Unfortunately, (fortunately?), we have had such gorgeous weather this summer/fall, that outdoor projects keep taking precedence over the bathroom...though I do see an end in sight. Looking forward to seeing more of your progress!

Raehelen, my cement board is going to be on hold now as I ready the floor for my delivery of a new Miele W/D set :) Looking forward to this. Sounds like your husband does thinks right. I think I can make the patch work. The thin set will be embedded with mesh tape. But, I will wait to make a decision in a few weeks. I might be energized by then. (see off topic note below).

To ready for the W/D, I have to paint and grout. I have a beautiful 12x24" highly variegated floor that I put in. I have Spectralock epoxy grout in "Dusty Gray" that I want to get in tonight. Tomorrow I will prime the walls and maybe even paint. I have all my supplies, it is just a matter of staying on task.

Tomorrow I will make a platform for my W/D out of 2 pieces of 3/4" Sturd-I floor with a piece of stall mat (clean :) sandwiched in between. This will be to help with potential vibration that some people complain about, as well as get it up off the ground a few inches.

Off topic note:
I have been very busy these last 7 days nursing a very badly injured 4 year old mare. She turned the corner this morning I believe. She wont be out of the woods for a month or more - is my guess. Injecting her with a total of 100mls of various antibiotics each day is a real task. Wound care. Pain medication. And TLC. It has been one awfully emotional week! I might nick name her Lucky, because she is, so far, surviving this ordeal. Maybe that is why my name is enduring :)

Can I chime in with related topic and see if anyone can answer?...my contractor too used drywall screws and taped the horizontal seam and filled in edges of niche with drywall compound. I bought hydroban to put over it all assuming that will solve the problems he's created, but then read it won't adhere to joint compound. So I sanded down the center seam and niche edges and skim coated with thinset...now planning to use the hydroban and move forward...does this sound acceptable? (I'll go on an replace all the visible screws first as I now see this is an issue. please advise!