Descaling a top loading washing machine without a built in heater
I think using a bucket of hot water would be fine, just don't use water over 60 C (140 F). In my experience descaling a tea kettle with white vinegar length of time is a factor. You might pour in the descaling solution and let it sit for an hour or two before you start the cycle.

I do not think you need hot water for the rinse.