Can you make the filling ahead of time, i.e., more than an hour ahead?

i agree. it's the ground pork you'd worry about. But imo, you could make the filling 2 days before making the gyoza.(the soy sauce and ginger are both preservatives.) When working with raw meat and/or seafood or poultry fillings, it is good to put their bowl over a bowl filled with ice. Also, leave in the frig your bigger amount of filling, removing more as you use it up.

Absolutely! No reason why not--obviously, just keep it refrigerated until you're ready to use it.