I am making a 10"-6" (2-tier) cake with a fondant bow. I have a couple of questions and would greatly appreciate any help, I have one day left to finish this cake.

Batter: I noticed the cake batter for the top tier was very thick. It seemed to bake up fine, but then when I went to carve off the crust I could tell the cake was crumbly. Almost as if it had alot of air bubbles in it, I can press down on it and you can tell. Will a "airy" cake like this hold up a fondant bow? I am contemplating if I should re-bake the top tier and make the batter more runny/normal.

Support: Also, do you need to dowel the top tier to support the fondant bow? Or should the cake be strong enough to hold it up?

Boxes: Any suggestions to where to buy a 12x12x15(ish) sized cardboard box, without having to order it online? Would a Home Depot/Lowes type of store carry one? My cake store only has 12x12x5 boxes, I need something taller.

Fondant bow: One more question. This is going to be my 2nd fondant (only) bow, so I am a little nervous of breakage. How long should I let it dry before putting it together? I made the loops 3 days ago. Is this long enough to dry?

Thank you!

First of all, use gum paste instead of fondant for your bow. You can roll it thinner, it dries faster and is a lot lighter than fondant. I use Wilton's pre made gum paste for all my bows and flowers and other types of decor that needs to be sturdy.
Just use a 40% off coupon from Hobby Lobby or Michaels and save a little money.