Hi oli...you'll need to make some of the calls.
1. Before knowing how much butter you'll need, you need to know how many cups of crumbs you have from your leftovers. For that size pan, I think you'd need around 3 cups of crumbs.
2. Just buzz the cake until it breaks into smaller pieces so that the melted butter can be easily blended in. Depending on the cake "crumb" (is it dry? is it really moist?), you may need a little--or a lot of--butter. I think somewhere between 1/2 cup to 1 cup of melted butter would do it. Start with the lesser amount and blend until the crumbs stick together when pressed then stop. Otherwise it will end up greasy and the butter will ooze out. Since this started out as a cake, you probably won't need any sugar. Taste to be sure. If you think nuts would work, chop them up finely...you don't want big chunks here.
3. Butter pan and gently press down buttered crumbs to compact...like shortbread. I like to put the same size pan down on top and press...that way it's all even. Bake at 425 for about 5 minutes to set.
For the creme, I'd go with 8 oz of creamcheese and 1.5 C of heavy cream, 2-3 heaping TBL of powdered sugar and some vanilla. TASTE and adjust since you'll be putting it on top of sweet jam. Be sure the creamcheese is beaten sufficiently so it is creamy before folding in the whipped heavy cream.
Your pan is slightly bigger than what I'm used to...is it a jelly roll pan?
Geez....now I want to try this. I JUST THREW AWAY a 6" Devil's Food CI cake yesterday because it was only so-so and I didn't want to be tempted to eat it.