OK, so this is really late, and the cake has long since been made and eaten (looked delicious), but here's a little input, perhaps for others who look at this in the future.
I also worked in an ice cream shop, Baskin-Robbins, for a while as a cake decorator. Dealing with frozen buttercream does add an interesting element to the whole undertaking.
Baskin-Robbins ice cream cakes are cake on the bottom, ice cream on top, and I have always preferred this ice-cream-to-cake ratio (about 50/50) myself. They are always covered in softened ice cream, sometimes chocolate, but usually a special "deco" vanilla that is extra white.
The piped decorations are generally buttercream--one of my favorite things about ice cream cakes as a kid (and still, as a matter of fact) has always been the dense, grainy texture of frozen frosting! Although the content of the buttercream may vary from shop to shop (they're franchised by independent owners), the stuff in my shop was (I believe, although I wasn't as savvy about these things at the time) of the shortening-based variety, in huge tubs from Westco, if I remember correctly.
We also used whipped dairy product (that stabilized stuff sold in quart cartons, I can't remember the name), which they whipped to a texture that could never produce smoothly piped shells, then stored in pastry bags in the fridge. And we used quite a bit of "dipping chocolate" (a la the ubiquitous Dairy Queen dipped cone, but put into a squeeze bottle), dripped along the edges or drizzled into designs.
The one decoration material that was a surprise to me was fudge. The same fudge they put into the heated containers to pour on sundaes, they put into pastry bags at room temperature, and it had a good, thick, frosting-like texture, excellent for piping, although a little stretchy.
As for the buttercream transfers, I did a few of those, and they are way easier than they look! Really great, especially for frozen cakes. We just put a sheet of wax paper on a template, piped buttercream to trace and fill the picture, and stuck it in the freezer to harden. Once it was hard, we put it frosting-side down on the cake, and peeled off the paper. The design comes out a mirror image of the original, so you may need to plan ahead for that. Really a fun technique.
Finally, here's a method I used for an ice cream cake last summer. I had made a shaped cake (uh, for a bachelorette party) which I cut out of a rectangular jelly-roll-pan-sized cake. With the leftover scraps, I made the base of an ice cream cake, pressing them into the bottom of a springform pan which I put in the freezer. Then I covered the cake (chocolate) with fudge and almonds, followed by slightly softened espresso gelato, and back to the freezer to harden. I decorated the top with some really good fudge, which I put in a decorator bag and piped in rosettes around the edge and a happy birthday message in the middle (obviously unrelated to the bachelorette party
). I also sprinkled some more almonds near the edge. It was really good, and really easy--kind of a frozen trifle. Also, because the cake was pressed firm, and there was only one layer, it cut really nicely.