Okay, let's see... No idea what Empire's opinion is on controlling multiple characters, but I'll take a look at both of them anyway.

Joshua: The stuff regarding Alex, I'll assume was discussed with Empire beforehand. As for the Gem-Knights... I honestly don't really know them too well, outside of an abusable loop they can perform with Gem-Knight Fusion. I assume your deck isn't built to take advantage of that in any way, though you could maybe replace at least one copy with something. Probably not necessary, though.

Amelia: Honestly, I'm not really sure how to feel about her backstory. An eleven-year old that's been hiding away with someone for an entire year within a dangerous district just seems... off to me. I'll need to let Empire decide on that one.
As for her deck... from what I remember, Gustos on their own aren't an overly-explosive deck, since they rely a lot on stalling and recycling cards to the deck. Shouldn't really be a problem in that regard.