As for the time, it should really matter but I suppose it also depends on whether you cross at a busier or less busy border post. We crossed at Ogdensburg, NY, at 11:35 at night and there were no other cars behind or in front of us. If they'd wanted to do a thorough inspection, they had enough agent available, but they didn't.
Just remember that she is only coming for a visit for now. They will decide how long that visit will be (probably based on however long they think the application will take, and considering that you can request an extension). In all likelihood, they will issue a Visitor Record for 6 months or less.
So, you can bring however much stuff would be suitable for a 6 month visit. Bringing either more or less than that could raise concerns. I just crossed the border with my partner last week. They asked what we were bringing. We told them it was mostly clothes and food. We did bring some family photos and such and we had the car stuffed tightly with as much as we could fit into it without blocking the windows. We were worried that they would pull everything out to inspect it, but it turned out that they didn't. Carefully cramming it all back in would have been a big job. We did have two "small" U-Haul boxes full of canned and boxed food from the cupboards and some stuff from the fridge.
The only thing they were concerned with, in our case, was whether we had any furniture, so I would advise against bringing anything at all that could be described as furniture. The main thing is to be concerned about is not to give the impression that she is moving permanently, because, for now, she is only visiting and must go back when the visit ends or in case the PR should be rejected for some reason.