Hi The least you should do in your case , is use liquid Waterproofing Membrane ( look at your local flooring supply store) it is kind of heavy and shipping cost will be too high, What I would recommend you to do is to mix a scup of no 20 or 30 sand with a 2 gallon of the liquid Waterproofing Membrane, it should give you great protection and easy to apply tile on. A product I am familiar and recommend is pro red by c-cure ( link to c-cure pro red attach) I would not recommend sheet membrane, you will need to know how to use it in/out side corners and overlapping and making sure it seal well., a risky way for leaks. The liquid apply same as you paint 2 heavy coat will do the job. Never heard of Hydraflex, I will check them out Here is a link that might be useful: C-Cure pro red

My greater concern here would be "Dynamic Pressure" which is the working pressure under flow. This bathroom is on the third floor so if this structure is on slab the vertical rise from the water main to the shower mixer will be approximately 24' and with a basement it could be as much as 32' or perhaps more. That would be a 10.3 to 13.8psig loss to vertical head. While the code required minimum dynamic head pressure for a standard shower mixer is only 8psig a temperture controlled shower mixer is required a minimum 20psig dynamic pressure. This means you would need between 30.3 and 33.8psig dynamic pressure at the main under full flow.

I used to live in Austin and desperately needed a softener there. We were concerned about that slimy feeling too, but you can control how much salt you use and we couldn't even tell we had one. It made a world of difference in everything else though. You can also look at water conditioners. People swear by them, and with one, you don't need softener. I'm still trying to figure out the pros and cons of each.

Not much info, and it's kinda a loaded question. I'm guessing 4Bed/1Ba 2nd floor, Powder 1st floor, and 2Bed/1Ba basement that were added later and are questionable whether they qualify as actual bedrooms. Agree 1 overly-large disproportionate bathroom on any floor is a waste. And having, if my layout assumptions are correct, 2 bathrooms in the basement, when you do not have a Master Suite, is very odd and will be a detriment to future value. From my experience in the market for large families, the sweet spot is 6Bed/4.5Ba or 5Bed/3.5Ba. Having said that, you should provide the amenities that you are comfortable living with. Most commenters here would not be comfortable living without a Master bath after having spent the money, time, and trouble putting an extra one in the basement that is associated with small bedrooms.