whats the highest possible distance you could fall into water if you were doing a back/front flip and most likely wouldn't land correctly
Regardless of your chances of survival, I wouldn't do it. If a body slap hurts as much as it does at 20 ft., it's not going to feel much better at 80+ ft.
What's the least dense liquid that isn't toxic and won't kill me if I jumped into it from 90 feet?
That question is nearly impossible to answer except in general terms: almost anything that isn't water and isn't more viscous (alcohol would do nicely, though that's toxic) could work.
The reason water is so deadly is because of two properties. First, water molecules are very strongly bonded to each other which is why you're able to "slap" water -- they create a layer on top and within the water that resists change. Second, water is very incompressible which means that hitting it at high speeds is similar to hitting concrete; the water molecules don't move much of anywhere when you hit them.
Thus, any molecule that isn't polar, can't hydrogen bond, and isn't large enough for London dispersion forces (the three main intermolecular forces that give water its properties) or has those forces but in weak magnitudes will probably be safe to jump into.
