During the busy season absolutely. And it's probably good to make them a little in advance. Shoulder seasons it's probably less important. I don't know when or if they close during winter.
I didn't have reservation. I was just there 8/16. But its always good to do so. Here's info on reservations. https://www.nps.gov/applications/glac/cgstatus/camping_detail.cfm?cg=St.%20Mary
Usually, yes
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