Yep, I have only one month in Canada but I do have H&C reasons. I shall bring the respective documents with me. Definitely I am in breach of RO. If it is not possible to reach on time, then most probably I will renounce my PR and reapply from scratch. Though it might take one or two years, it is worth doing it because Canada is one of the best place to live and work.
One month+18 months of credit (assuming you move by end of year) will give you 19 months. You will in breach by 4 months.
Try to go ASAP, emphasis on ASAP. A 4 month breach is not as egregious as it gets, so I like your odds at the airport. Do not renounce your PR at the embassy.
My recommendation is as follows:
Fly to Canada, use automated clearance.
Have a clean customs declaration, and pay attention to the little things, i.e., dress well, act calm, don't have "Send me to Secondary" written on your forehead.
Hopefully you are waived through. (This is where delay comes into play. Pick a flight that lands at 12 noon instead of 1 in the morning.)
If you are not waived through, explain your H&C reasons to the officer in Secondary. Say it's only a 4 month breach, you needed some time getting ready to move, and that you are coming for good. Say whatever else it is you feel is relavent.
At that point you will either be waived through by the secondary officer or be given the option between renouncing your PR or getting issued a report.
Assuming you still qualify for Express Entry and assuming you don't have very compelling H&C reasons (btw what are they) then renunciation if the alternative is a report may be suited for you. Renunciation still allows you entry into Canada, but as a visitor and only for 6 months.
Alternatively you can take the report, try your chances with the Minister's Delegate, and if it doesn't work out and you are issued a departure order you can appeal. During the waiting period for appeal you can of course work etc as you will be a full PR.
The thing to note here is your age, because as you get older immigrating gets harder. Assuming you are in your mid 20's this wouldn't be a problem compared to say someone in their 30s who barely qualified.
Assuming you don't want to deal with the hassle of an appeal and face the possibility of leaving Canada then you may choose renunciation; however, if at all you renounce it should be at the border.