Crumbly texture-scratch cake? Underbeaten; too much shortening (according to the cookbooks that I own).

Personal experience:
If you used margarine instead of butter, was your margarine 100 per cent to at least 80 per cent fat?
Margarines have water content and WILL affect the cake
or baked product texture. Read your label.

I prefer to use yellow or golden cake mixes when decorating cakes, (especially, wedding cakes.)Whole eggs help the cake texture hold, helping the decorator to decorate better. Make sure the cake is totally cool
before decorating. Placing a cake in the freezor for
a short time (15-30 min) helps to firm the texture for easier decorating. Brush off all crumbs lightly, before decorating.

I have successfully used yellow cake mixes and the flat bottomed ice cream cones and decorated said finished products. Fill the cones about 2/3's full, bake as
directed on package. Watch carefully, to prevent burning. Cool, chill, decorate.