How likely is it that the other Northerners realize that Roose Bolton is sending them to their deaths while keeping his own men back? I get that at any one battle it’d be easy to explain away, but after a few, surely someone would notice that the dreadfort is not taking casualties?

Good point. I think the secret to his success is making sure the people who would catch on don’t make it back. Hence, making sure that Halman Tallhart and Robett Glover don’t make it back to report to Robb and find out that they were told Roose had orders from the King in the North and Roose told Robb they went AWOL.