12x24 can be a challenge, even for a pro. I'm assuming you're going to put them in a brick pattern, and if so, a word of caution: Those size tiles can be crowned sometimes, so you might look into one of the leveling systems, like Tuscan or LASH to keep everything flat. Tuscan is probably a better system, but has a higher startup cost. With the LASH you can by only what you need to complete the job, and only be left with the spikes at the end. Or buy the Tuscan and when you're finished I'll buy the gun from you for 10 cents on the dollar.
You can certainly install those tile on the wall, but get some solid spacers for that. Most of the soft rubber ones will crush under the weight. Look at the multi-spacer http://www.multi-spacer.com/
or the Roto Wedge http://www.advancedtiletools.com/wedgevideo.html.
Again, the leveling systems are a big help.
Ultraflex 2 or 3 are great products. Most importantly, skim thinset on the back of each tile to obtain maximum contact. It's especially important with larger porcelain tiles like you have. You can use the same thinset on the wall. Say "NO" to the premix stuff. 1/2" square notch is okay, although I like the 1/4x1/2x1/4" trowel. It's the same amount of thinset, but the notches are closer together.
Expansion joints must be honored through the tile. You have a couple of choices. You can by metal expansion strips through Schluter (or wherever you can find them), or you can use a color-matched caulk to fill those joints, which should be the same size as the grout joints so they'll blend in. If you're lucky, the expansion joints will be square with your tile layout so they won't stand out so much.
Mapeguard 2 would probably work, although I have no experience with it. I use the paint-on membranes like Redgard or Custom's Fracture-Free, which is cheaper than other crack-iso systems. Any anti-fracture is going to be compatible with thinset, to my knowledge.