mixing dry ferts in hot water

It should be fine, but I find that KH2PO4 and KNO3 dissolve quickly in a mug of tank water (normal 20-40g EI dose) ...5-10 mins anyway. CSM+B takes closer to 30 mins. Either way, every morning I scoop up a mug of water, measure out the chems (alternating macros and micros), and let it sit for a while. I come back later, give it a final stir, and dump. That way I don't have to make a trip to the kitchen.

I mix mine in a couple of jugs (Macros/Micros), and use hot tap water. Not so hot that I cannot touch it, but pretty hot.

There's no point in mixing it in water first just dump the correct amount directly into the tank. It's easier and faster and won't make a difference.

I quit pre-mixing my ferts years ago and haven't seen a difference.