I need to do a cake for my SIL's birthday on Sunday, but this week is jam packed for me and the cake needs to be ready early Sunday morning. I was thinking about baking, filling and crumb coating the cake today, letting it crust overnight, and then covering it with fondant tomorrow. And working on the decorations when I get some spare time here and there.
Do you think the cake will be alright? The filling is shortening based, so I don't have to worry about that going bad, and I'm thinking the fondant should keep the cake inside fresh... right?

I work another job, and always have to do my cakes on a week time-line. I almost always prepare my cake the beginning of the week and work on decorations and things as I go along.

As long as your crumb coat is bc it will hold up in the Freezer for a few days. I don't know if the crumb coat would be enough icing to keep the cake fresh for a day. But once the fondant is on you should be good until Sunday.

But I am no master baker, just a hobbiest

Bake away, your cake will be just fine for Sunday

I also work nights so I only have day time hours along with the rest of my life to do my cakes. It alwasy seems to take me 3 days to do any cakes on my list. I always bake my cake and sometimes refrigerate till next day than crumb coat and or sometime ice depending on time and than decorate the 3rd day this also includes fondant, it always keeps no problem. I always have cakes ready for the day before delivery so that way if my schedule changes or my customers changes than it is ready for pick up at any time the day. So you should be fine to take your time.