There is nothing wrong with blending a little water from the hot water tap when doing water changes.
When I lived in Milwaukee my tap water in winter was often below 40' F and only 50+' in summer, so I blended the hot cold for decades without a problem.
If you are concerned about mineral buildup from the water heater, drain some out of the heater unit from the low tap, until it feels like water (not viscous).
I found by doing a draw of about 2 quarts from the heater every 4 - 6 months, it kept mineral down.
Your water heater instructions recommend this anyway once or twice per year, to improve efficiency.
The heat of the water does not kill ick, it just speeds there life cycle allowing the salt to work.