Donald wanting to be a guard as an adult as a child makes sense to me. I get the impression that Donald thinks of himself as a hero, who would be able to defend any place from anyone... even though that doesn't necessarily correspond with reality. And there's also the fact that Barks employed him as a guard at museums and stuff on numerous occasions.
Judging from these panels, it actually seems like this story is intended to take place before HDL arrive, which we don't see too often! A pre-status quo story! But as you say, that doesn't make sense in the timeline. I could buy that the guard job hasn't been available during Scrooge's withdrawal in between 1942 and 1947. Donald would probably put this dream to rest after his family cut ties to Scrooge. But there could be revived interest in it post-Christmas on Bear Mountain. I seem to recall that my head-canon places The Big Bin on Killmotor Hill soon after Bear Mountain, when Scrooge has opened the Bin for business again... (The "New Bin" would only refer to updated and improved security and things like that.)
But then HDL should still be present, so never mind.
Well, it's worth pointing out that the comic doesn't have any scenes at Donald's home. Most if not all scenes are at the Money Bin, if I remember correctly, so the story could've happened while Huey, Dewey, and Louie were at school.
No matter what I say or do, know that Jesus loves you.