pre-arrival letter - it means that the PA has met all the eligibility requirements for permanent residency and you will soon be getting a final decision.
I do not have an answer for you but I wanted to congratulate you on how fast your application is being processed. I wish my file had stayed at the Mississauga office. Does your mom have a super-visa?
Processing time dropped to 20 months today. May 19th will be 17 months since I submitted my application. Hopefully, I'll see some action from the Montreal VO soon.
I thoughts the GoC was working on improving processing? Unless it's for all other IRCC programs. Processing time is increasing steadily. It's now at 27 months.