You can
tile on old plaster providing it's solid enough to take the weight of the tiles. So if it's flakey as in just a bit dusty flakey then you're ok providing you follow the normal tiling practice but if it's flakey as in falling off flakey then you need to sort it out.
I'd probably dot and dab plasterboard on the really dodgy walls and then use a but of the dot and dab mix to patch up the not so bad walls and
tile it like that.
But the unsquareness of it isn't anything new. You'll never find a bathroom that is square enough to say it's perfect but you can
tile it perfectly no matter what condition it's in.
You need to set out well. If you have a big gap at the top and your set out allowed you to go full
tile at that point you'd simply be trimming the tiles as you go down the wall. But if you set it out wrong and had a full
tile at the bottom you'd obvisouly end up with a gap at the top.
So you need to set it all out and get the best looking setout for all the uneven walls etc.