Can undue hardship be applied retroactively?

Undue hardship can be argued if your situation warrants it but to get it applied retroactively you would need to present yourself with “clean hands” and demonstrate that the hardship was beyond your control. In essence, you will need to demonstrate that your employment loss was completely out of your control and that you had no other means of paying the ordered support. If your change in circumstance is legitimate, you might not need to plead undue hardship and might be able to have the court revise the ordered support retroactively to the date when your situation changed.

As far as paying arrears a little every month, this depends on your actual circumstances and needs to be negotiated with either your ex-spouse or Revenue Quebec.