Am I missing something?
Your problem isn't the wall or tiles, it's the deflection in your joists. Reboarding, tanking, re-tiling won't solve your problem because as soon as you walk in to that shower again the tray will move and break any seal you have just made.
Is this on a first floor? If so can you tackle the problem from below? I'm assuming so if you've noticed the leak.
Remove a section of plasterboard from below and shore up the joist by adding extra noggins, joist hangers, slate packers... whatever you have to do to lessen the defelection.
Get the other bathroom finished then get a dehumidifier in the shower and let it dry out, then just simply resilicon the shower as it is without retiling... unless of course the tiles are falling off the wall? Or maybe just remove and re-tile the bottom tiles? Not sure how bad this leak has been?