My Breville Dual Boiler has been acting up lately-making more boiling/hissing/refilling noises. There are also some mineral deposits around the steam handle.

Before Breville will start the warranty process they asked me to pour descaling solution in the tank, run it the group head AND let it build steam pressure. Is this ok/safe with the descale solution? In particular, is it OK to create steam using the descale solution?

Bump-edited for brevity

Avoid breathing a lot of it, but yes. If Breville says to do it, go ahead. I try to stop heating descaler before it boils, but I've boiled it more than once, it cools back down. It is most effective when hot (but before actually boiling).