An all weather outdoor shower is easily done by locating the mixing valve inside the house and use a coupling shaft to locate the handle on the outside. You must use a standard tub type lower faucet (with the pull up to activate the shower) to allow the system to drain out when the shower is done. You also have to make sure everything slopes in direction of the water flow. We live in Bancroft Ontario so winters are well below freezing and we have been showering outside for 3 winters. Only issue we have is the shower head will freeze so we typically remove it after a shower or shower without it on when really cold. The system has worked well and it helps that nothing special needs to be done by anyone. no inside valves to close etc. Just turn off tap and everything drains.

I have a similar sized ensuite in my own house and would definitely not put two doors in it. You will probably find that a fixed glass panel to half the width and a glass door the other half would work fine. Alternatively you can get a sliding glass door (again 50/50) if you like the look of them as they do save on space.

I have to ask, do you really need that bath when you have such a tight space? Unless you have tiny children (and remember that's a temporary situation!,) I suggest a larger walk in shower e.g. 1200 x 800 and heated towel rail on the wall nearer the window so its more convenient. Would definitely keep the double vanity for those busy mornings with storage under and you could add mirror fronted cabinet recessed into the wall above the basins to add more storage. Hope that helps!

My comment would be that fluted glass would be a bit harder to keep clean??