Personally, I think it's probably the Splenda. Something that is moist yet crumbly sort of speaks of the texture diff between sugar and slenda.
A lot of flourless recipes call for baking powder as leavening, so I don't think that's it. And I don't think expired BP would make something crumbly (check it by adding some to water -- it should fizz)
They make a Splenda brown sugar blend, which might work better. But would be more calories.
Less is not more. More is more and more is fabulous.