Protection from weather damage once drained? I don't think you have to worry about that too much - there's ALWAYS water in the ground that's surrounding the pool, therefore, those walls more than likely have had water on them for the majority of the 40 years they've been there, a couple of days/weeks of exposure shouldn't do any more harm than what 40 years have done.

With the length of time this pool has lasted, I'd say you may just discover some things that are going to turn you off repairing it anyways - that's an awfully long time for wood to be under ground - and those supports behind it may well be on their last legs all around and not just in the weak spot - the only way to tell that is to remove all the walls......