For irons.... I have found a heavier base iron seems to work better for snowboards at being about to consistently melt and distribute the wax.
I had a Swix T75XF, which I had a struggle a bit to apply the wax. (works fine on skis).
This year I bought Toko T14 iron which is a lot meatier on the base, which works a lot better.
To answer the other question, just keep applying heat through the iron until the base heats up (when the top sheet is warm to the touch, is your indication), of course, gotta keep the iron moving.
All-temp waxes usually melt at about 130°C.
when you hit the 140°C mark, that is danger territory for damaging the base.