Been there and it stinks, especially losing two courses in that heat. Ninety degrees in the morning is brutal, and that pallet baking in the sun all week probably made things worse than you'd expect. Hot sand plus hot tub plus hot water means the mortar sets up way faster than the mix ratio would suggest. I'd bet on the heat, not your recipe. Try mixing smaller loads and keep your water in the shade if you can, cold water buys you a few extra minutes. Splitting the wall into shorter runs sounds slower but beats tearing out block.

Forty percent?! Stop it, that can't be real.

Maybe it's just me, but check the adhesive on damp concrete before you trust it.