You can stay out for just 5 minutes but each entry is entirely at the discretion of the Immigration Officer holding your passport. It will be obvious what you are doing whether you stay 5 minutes or 5 days having just maxed a Visa Exempt entry stamp.
Yes, probably right. There are two 90 days, one is the 90 day validity of the Non-O Visa from the date of issue then the 90 days entry stamp given when entering during the validity of the Visa.