Your recipe is heavy on the butter - quite rich! You can whip in butter, shortening (nontransfat if you prefer), or just more confectioners' sugar. If your mixer bowl is large enough you should have no problem. When I make buttercream for my family, I don't measure and don't follow a recipe. IMO it's impossible to mess up American buttercream.
It's possible that the cake could have remained in the box "exactly the way" you boxed it. When I see those forks (they're forks, right?) my imagination says they looked into the box, saw a cake in pieces, and got hungry. Hence, forks in box.
Just asking. This is not a business for you so I can see where your concern is limited by personal constraints. Still, I'd be concerned about the issues I mentioned above. I suggest you bring a kit when you return for the event. It never hurts to have extra buttercream, tools, etc. Good luck and enjoy the wedding!
I suggest you write a short business plan that covers these question. You will have a lot of money invested in inventory. If I was in your situation, my focus would shift from sourcing products for resale, to customers/sales/volume.
I have a convection oven but usually bake without the fan. I'm pretty sure this topic has been discussed a few times. It may help to search for "buying new home oven" or whatever keys words will work for you.
And P.S. - For the best thread ever, visit Found hair in my oven. Don't forget to vote.