Mitel will not reset the password for an end user, you need to get a vendor with a Mitel SX-200 ICP certified technician to take control of the AMC record, there is some paperwork for the vendor and a fee. After they are in control of the application record (AMC record), then they can file the paperwork to get the password reset. There is no quick and easy way to reset the password on the 200.
Did your tech try maint1 and 1000? Maint1 login usually has most of the capabilities of Installer, except software upgrades and to reset passwords for installer, but few vendors ever change that one.