Hey guys.

Since we're in the midst of a pandemic, I've decided to renovate my condo and am serving as my own contractor. I've replaced my floors (vinyl luxury plank), sockets (usb outlets), light switches (everything now white, not beige), kitchen light fixtures, cabinet doors and hardware, replaced and installed my stove, and have painted the entire place (walls, baseboards, and shoe mould). I've installed my own toilet, new shower valve and trim, replaced insulation, and built walls for a bathtub that I paid to have installed, which wasn't installed correctly, then diagnosed and sealed my overflow drain which was causing a leak in the garage (I'm the terrace level, one floor above). I say all this to demonstrate that while I am a newbie, I am also resourceful, determined and stubborn about learning and doing this work correctly.

Wall/waterproofing issue:

The walls that I have built and half waterproofed with schulter membrane are made of durock, and sealed with thinset. While watching Jeff Thorton on his youtube channel renovision, I realized that I may have waterproofed wrong, he recommends unmodified cement, not modified like the one I have which is for setting tiles. The other problem is that the walls have a bit of deflection, like 1/6 of an inch maybe, with the joining seam having been built between two studs. So now I've taken off the membrane which peeled off pretty easily and believe I have a few choices.

1. reinforce the deflection with metal stud framing and membrane the Durock walls I have with unmodified concrete
2. take down/demo the walls I have and start over with schluter kerdiboard.

In the youtube videos I've watched, Jeff Thorton advises to start laying the tile then cut out the niche on a grout line. The kerdiboard (which originally seemed crazy expensive) now seems like an easy to install and light weight option, totally worth the money, for future niche prep.

Niche issue:

I have this idea of doing a long niche along the back wall that I can't let go of. Currently I have built two square niches on the left wall, opposite the shower valve and faucet wall, one inside and one outside of the tub. I like this because water collection will be minimal. But I really love the aesthetic of the long niche and want to do that instead. The problem with all niches is that my condo has steel studs, all about 2 1/2" wide, so its a shallow niche, 3" total with the 1/2" Durock on top, with no durrock on the back. To make the long niche, I'll have to cut out one of the studs on that wall and brace it with more metal studding. Additionally, it is located on a common wall with my neighbor, so I'm worried not only about the increased water collection but the potential damage to their condo should something go wrong. I don't believe there is a fire wall or any extra spacing between our apartments.

Can anyone advise?

1. Should I demo and rebuild my Durock walls with Kerdiboard? Or should I stay the course with my Durock.
2. Can I build the long niche on the back wall like I want? If so, what can I do to make sure there are no problems with my neighbors place?

Thanks in advance for your help. Pictures attached.