On day one of indoc, they used to say, "if you lie to the company, you will be fired". Same thing for cheating on a travel claim.
If you don't lie, or answer questions untruthfully, you probably won't get fired.
It would have been better to disclose the letters and when asked in the interview, give your side of the story. Of course, it is too late for that now.
Remember, if you are hired, you are on probation for a year, and can be fired during that year and the union pretty much can't do anything to help you.
The lesson learned is to answer all questions truthfully.
I am sorry it was such a hard lesson, but perhaps someone else can learn from it.