You have done extensive troubleshooting, and pretty much eliminated all possible sources of the leak. I do not think it is plumbing related, based on your all the investigative demolition. (How much has that cost?) This is a bad tile job. The next step is to tear it all out. Right down to the studs. Examine the tub installation: Is it level? Is the rim supported to the walls around the perimeter on all sides with solid 2x4? Is the bottom supported continuously to the subfloor? Now look at the tub tiling flange: Is it flush with the wall so tile can go over it? Is it tall enough to contain the water? You may need to have a new tub installed, or the same one reinstalled properly. Next, cement-based surfacing for the walls: Hardiboard, Asbestos-cement board, or mortar bed, is industry standard for wet areas. Do not use gypsum paneling, even if it will be coated with waterproof membrane. Install waterproof membrane, like Kerdi, set into a layer of thinset. Make sure the membrane covers the tiling flange. Install tile over the membrane, after it is completely dry. Wait a few days before grouting to allow all the mortar to cure completely. Waterproof membranes really slow down the thinset from curing. UN-modified thinset is required.

I don't know if it is true but I have heard that it is not good to use all tile in a pool in climates with such diverse temps. I would look into the Pebble Tec line of products. They are very strong. All tile pools are usually seen in warmer climates. BTW-I am not a pool expert by any means, so if I am wrong I am sorry, but I thought there was a thread about this in the past.

If you want something that is waterproof, they do make AquaGuard or NuCore laminate flooring that is made with recycled rubber. You could also go with a wood look tile option. But you could never really go wrong with a hardwood floor to increase the value of your home. Just throwing a few options into the ring. Check out Floor & Decor if you're interested in a waterproof LAMINATE. - Woodruff Floors, LLC

I'm guessing you have additional tile left over. You should anyway for future repairs if needed. My suggestion is to make up 3 "panels" of the tile on say a piece of plywood. Whatever thin set color was used to apply the tile to the spa should be the same. With the tiles being a bit transparent, the thin set color will show through. Grout each with the different colors, and allow them to fully dry. Place all 3 next to the spa in the daylight, and exam the color differences. This should help in your decision making. The coping will cast a slight shadow at the top of the tile so have someone hold the tile panels against the spa, under the coping, to duplicate the conditions. Consider your plaster finish coordinating when making this decision also.