proprietary software used to program this system and how can I acquire it?
yes, MiVoice Office 250 System Admin & Diagnostics it is available on Mitel online
is there a way to find this without defaulting the system?
Mitel has a backdoor that they can typically enable but at a cost... something like $500 one time
SIP Trunking: is the system able to accept SIP trunks?
Yes, there are several approved carrier and others which are known to work