Geez, where to start...
I think your major problem has been filing a 1040NR. Even if neither of you have been living in the US, you should have been filing a joint 1040. You are allowed to by virtue of being cdn (and also by virtue of having spent enough actual days in US every year).
This would have lowered your US tax rate substantially, and this would have made RRSP worthwhile (and deductible) in canada.
If your husband has been working in US, why would he now need an ITIN? Does he not have an SSN right now? It is good forever as his tax ID.
There is no link between your HR and your withholding tax and your RRSP. the link is in the manner in which you have been filing your return. You have, in effect, made your US tax rate higher than your cdn tax rate, penalizing yourself by not using all the tax deductions available on a 1040.
1040NR was the absolute worst way for you to file

, even if your husband was always working in Canada, which he wasn't.
even with him now remaining in Canada a joint treturn, or at the very least a non-discrimination Article XXV return would make most sense.
Ordinarily, I would be going back as many years as legally allowed, and refiling as married joint 1040, but unfortunately for you, this would probably result in you taking all the money refunded to you and sending it to CRA, since you did not contribute to your RRSP when you should have.
But, ecertainly for this year, while you still both work in US, I would be filing a true blue 1040 and contributing just enough RRSP to reduce my Cdn tax to NIL.
After 20 years, I am severely cutting back on responses. Do not ask specifically for my help. There are a few others on this board that can answer most questions. All the best