Being in a similar relationship, I am a US Citizen and my husband is Canadian. We are a 45 minute drive from my door to his parents door and that includes crossing the bridge or through the tunnel. Our lawyer also applied for our AP for us. My husbands last border crossing was in mid 2005 when our daughter was born. From what I remember you are correct that things are different for US/Canadian immigration. Especially that we have that loophole that they do not have hard copy proof of when Canadians enter. This is why we applied for AP. Being a Canadian in the US with no proof of your last entry.
We have a lawyer as well, we were very worried about this as well but we were told that as long as we were married when he crossed the final time and they did not ask if we were married or anything else he was fine. The last time my husband crossed the US/Canadian border they didn't even ask him for his ID or anything. They asked him citizenship he said Canadian and they told him to go ahead. My husbands last entry being in mid 2005 we didn't file until this year and that was simply because we had to save the money (no, he has not worked at all).
I personally think its a matter that you need to talk to your lawyer about. Everyone has a very different situation and since you have a lawyer its definitely something to talk about with them. That is what you are paying them for.
But you are correct, Canadian American immigration varies a great deal from others.
TNGuy is right as well, you will need to prove yourself to USCIS when it comes time for your interview. In my case my biggest piece of proof is we have a daughter together. We don't have anything (bills etc.) in our names together, only because we are waiting for EAD to come so he can get things in his name but without a SSN# he cannot do so. We have tons of pics, wedding info, funeral information from my father, letters from family, etc. We are hopeful by the time our interview comes up that he will finally be able to put things in his name
I know its not a lot of help but I highly advise you talk to your lawyer directly. Also do a search, I have read on here from someone that you cannot accrue legal status in the US if you do not have a passport stamp, it's a well known catch that you can get around. But I'm not going to get into that, I'll let you do some searching and read up for yourself!