I applied the Mental Health Waiver but wasn’t admitted for in-hospital treatment (i.e. I didn’t claim). Does this count as my ‘once-in-a-lifetime’ usage?

No. If you weren’t admitted for in-hospital (inpatient) psychiatric treatment within two months of accessing the Mental Health Waiver, it won’t count.

That means you can choose to downgrade your cover safe in the knowledge that the waiver is still an option.