Hey Lyman! At 5' 6" you could definitely go either way depending on your usage intention. If it's strictly going to be for ripping groomer laps in the resort and then venturing off-piste as well, the 171cm would be a pretty easy choice. Though for a steep ski or mountaineering ski with some use in the resort as well, the 164 is the sensible choice and would still do really well in the resort though you may sacrifice just a touch of high speed stability. For reference, I am ~5'7" and ski the 164 Voile Object...
Hey Tim! Your guess is as good as ours, the pivots on the X-Alp/Proclines haven't been made available as a spare part so who knows if these will work or not. The safe bet is that no, they won't work.
Hey Evan! Potentially! There are about 5 bindings that have the designation "FT 12" so it's important to know which generation of binding you are aiming for. The "classic" toe pieces are the 5-Hole patterns like the Vertical, Comfort, TLT Speed, etc. The hole pattern on Dynafit heel pieces hasn't changed much up until just recently, but you definitely can't mount the heels to the fast plate though as those are designed for the Kreuzspitze heels.
Hey Mike! The Alien RS has a lot of potential to be a great replacement actually. I don't think the TLT7 would be a good fit for you. The Movement Alp Tracks Performance is pretty much the spitting image of the TLT6 in terms of fit. The Salomon X-Alp is another good one, but is closer to the TLT5 than 6, still fairly similar though. When in doubt, here is a direct line to our boot fitting fairies.
Hey Pico! Looks like we're out at the moment and probably for the foreseeable future. The good news is that we have a Used Dynafit Parts page that's worth keeping an eye on. Worst case, the Vertical heel hole pattern matches up with the Speed Radical and Speed Turn heel pieces so a full blown replacement wouldn't be the end of the world.