Engagement Cake: Design

I've started work on the cake I'll be decorating for my class this Saturday.  The cake will be a present to my coworker and his fiancee.  It will match their lovely wedding invitations ... hopefully ... that's the plan ...

Stats:

• 8" square cake
• buttery chocolate box cake mix
• chocolate buttercream as foundation
• coffee Italian Meringue Buttercream as filling
  
• covered in brown marshmallow fondant
• ivory square piece of fondant on top, painted to resemble their wedding invitation
• ivory roses with brown leaves adoring 2 corners
• shell borders
• cornelli lace on sides

Concerns:

• The 8" square pan I have is only 2" deep.  I'm not sure if the cake batter will be enough or too much.
• My husband remains skeptical that painting on to the fondant will look ok.  I hope to pull it off.
• I'm not sure if the shell borders will be pretty enough.  I do think they'll match nicely with the swirls of the invitation design.
• I made a half-batch of ivory fondant last night.  I hope that's enough for the roses and the square piece on top.  I'll bring store-bought fondant just in case.  I can make some practice roses with that.