I pulled off the bumper to ensure that it was stamped "MADE IN CANADA" along the top, and sure enough, it is:
After going back and forth, trying to decide what to do, I eventually just gave up and decided to mount the bumper to the car, with the poor fitting grill. I have plenty of other work to do on this car, and I didn't want this one issue to hold up the whole project.
Resolution
However, in the process of mounting the chrome bumper to the rest of the assembly, I had to force the bumper quite a bit to get all the screw holes to line up. Once I finished forcing everything together, the plastic grill insert lined up much better! Strange thing is, I needed to mount the bottom chrome bumper fasteners BEFORE the top ones, to get everything to line up properly. For whatever reason, starting with the top fasteners first doesn't work.
I'm now thinking that the chrome bumper got slightly bent out of shape from laying flat for 3 years in my driveway.
If anyone else experiences an issue similar to this, my advice is this: mount the bottom of the chrome bumper to the assembly first (but keep the fasteners loose), and use the 4 brackets to hold the plastic grill insert in place. Then, with the bottom of the chrome bumper mounted, use some force to get the top of the chrome bumper's mounting holes to line up. You may need a second second of hands to run the fasteners through the top. Once you have all the fasteners in place, tighten them up, starting with the bottom fasteners first. By the time you've made your way to the top, the grill insert should line up much better.