I have seen people get a rejection and raising a CSE with an attached missing document along with the letter of explanation. Some had their cases re-opened, and some did not... But you definitely should try raising a CSE and attach the job offer composed according to IRCC's specifications along with the explanation why you haven't done so before as soon as you can.
IRCC officers are not emotionally attached to cases (would be horrible for them if they were). They have specific guidelines for review of the cases (which they cited in the rejection letter), and if the application does not fall within those guidelines they may reject it. If I may offer a piece of advice, look again at what IRCC is looking for in the job offer letter, ask your employer to write you a new one, and submit it to IRCC. Just try not to panic and don't think IRCC is rejecting you on purpose.