Upvote:2
Why not state the truth, explaining the situation exactly as you have here: you have resigned, are working during the notice period, and will attend a job interview during your visit to Canada. Make it clear you understand what is permissable on a visit visa, that you may attend an interview but not work, that you have sufficient finances, and that you are to leave Canada before an entry with a work visa, should that be issued in future.
As for your documents, you can include the resignation email and the confirmation; it supports what you're saying, and clarifies why your current employer may be unwilling to provide any evidence of leave or employment. The old job offer letter is unnecessary, and irrelevant. More importantly, include any supporting communications, documents, or information from or about the Canadian company with which you will interview.