First of all, you still haven't specified whether your floor is "not flat" or "not level." Whatever else you have (or have not) done to get where you are, it would help a lot if you could say whether your problem is level or flat. Also, just to be sure here, if I am understanding you correctly, you installed the 2x4s ACROSS the 2x6s that make up the underside of the house? (Notice the very very NON-technical terms...so be aware that I'm not a builder or knowledgeable in this stuff like some of these people that post here...just sympathetic to rising note of panic in your posts....) The only reason I'm trying to clarify that is because I am wondering whether you installed the 2x4s "upright" (so the "4" dimension is vertical) or "laying down". I'm not trying to pile on to what you have *already* done, but I don't know what the proper distance for SPANNING that 2x4s have, and I don't know whether they can span two full feet if they are flat. This makes a difference because you want to install tile. If your subfloor is bouncy or bends ("deflects" I think is it), then all the tile you install will just crack and plague you and your heirs until the end of time...until you rip the whole floor and subfloor out again and re-do it. Or put in linoleum. Whichever comes first. You say in the top post that the cracks are like 1/4", but in your last post you write that they are up to 1". That's a big difference? Which is it? Does it vary from 1/4"-1"? Have you actually measure or are just eyeballing or going from memory? IF YOUR CRACKS ARE VERY SMALL: IF it were me (and it isn't), I'd probably just put down a very tight layer of Wedi board (thinset and screws/washers), seal the seams so it can hold the next layer, then put down a thin layer of self-leveling compound (though you still haven't said whether your floor is wavy or slanted), and then tile. IF YOUR CRACKS ARE 3/4'-1": Maybe use a layer of plywood (which would be cheaper but harder to cut), maybe tape and thinset the seams (ask someone on this board about this because I'm just brainstorming here), then use the SLC, then the tile. Only plywood will actually help form a floor (which I think you need to do with 1" gaps), not backerboard of any kind. Instead of Wedi, you could use probably any backer board (ask someone here what to do for the seams). But I only know about Wedi because that is what we went with, and I like it because it is light weight (good if you are a female working alone). I don't think your solution is in one step. It feels like you are going to have to create a tight subfloor, THEN level or flatten. I think you have a non-standard thing going on there, and that might not be addressed so easily with just one step before tile. One more thing, just out of curiousity, WHY do you think your floor must be level? Oh yeah, and I would start making your peace with a threshold. Given how non-standard your set-up is, AND the fact that this is DIY (or at least the subfloor), I'm not sure it is realistic to think you can get away with no threshold.

As a Certified Red Seal Tile Setter the substrates (sub structure) and prep work are crucial elements and have a direct affect on not only the finished product but also the long term performance. Since this project involves a kitchen back-splash, there is no need to buy/install cement board of any type or a thing of the past called Green board. Like any tile job, everything must be level/square and solid first! Removing existing tiles and minimizing the damage to the existing wall board requires the right tools and a certain degree of patience. Personally, I have never seen cement board behind any type/kind of back-splash tile(s). Ceramic tiles are the most porous kind of tile and should be properly sealed as well as all grout lines. Also, in water areas, your grout line thickness should never exceed 1/8 of an inch. This helps in minimizing water penetration and also helps in the visual appeal. The idea is to showcase the tile not the grout lines! Secondly, and equally important is to try and get the grout color as close to the main color in the tile, that way it gives a visual appearance in being one large piece of tile, although it isn't! Solid color tiles should always be complimented with a slightly off color grout (i.e. white tile - off white grout). Your investment is in the tile, not the grout line, so it makes sense to showcase the tile! For DIY people, forget spending your time looking at 50 different videos on YouTubes or even worse, the advise you get from some hourly clerk at your local improvement store! It is to your advantage to buy your tiles at a place that specializes in tiles only and nothing else! They will and should supply you with everything you will need material wise to do your intended project. On another note, buy a few tiles first; take them home and place them where they are going to be installed ( tape them to the wall if that is where they are being installed); stand back an look at it to see if you like the color/size and the pattern. What any tile looks like in a showroom can look a lot different under different lighting conditions!

"... if tiling up to the ceiling above a soaking tub (without a handheld shower head), can you tile over established drywall that has Kerdi applied over the drywall." For the walls around a soaking tub, if you want to hang Kerdi on the drywall and tile on the Kerdi, that's perfectly acceptable. "I had also thought you cannot tile over drywall." In non-wet areas, you can tile directly on drywall. Example, bathroom walls outside of the shower, or kitchen back splashes. You can't tile directly on gypsum board in wet areas like showers and tub surrounds, even if the gypsum board is moisture resistant. In a wet area, Schluter allows Kerdi to be installed over gypsum board. In that case, the tile is on the Kerdi, and the Kerdi is protecting the gypsum board.

I suspect you will regret bamboo. I have stranded bamboo in a kitchen and I wouldn't put it in a bath. If you really don't have the money to truly fix the deflection, use a small mosaic tile, hexagon or penny-round will probably work best. Use a masonry board or a uncoupling membrane and then adhere the tiles with a flexible thin set. I prefer Laticrete 254, but if you have to do the big box store, Flex bond from Home Depot isn't bad. Then use a decent flexible grout. The Mapei flexcolor is a solid product. If your floor is flexing enough to break those tiles then you have much bigger problems. Eventually the grout might work loose but you should have quite a while before that becomes a real problem. ETA: Check my advice on the johnbridge forums if you would like. You can literally paste my advice to your problem over there and get opinions from professional tilers who will put their name and experience right there in their signature.