To clarify the substrate and heating pad issue, basically the heating pads that are designed for human use are not made to be constantly on. Sure, the ones designed for reptiles can malfunction, as can any electrical device, but they are designed to be on for long long periods of time. There's not really a scientific reason for this, it's just what they are made for, but you are much more likely to experience a malfunction with the one you are using that one designed for reptiles due to the already mentioned fact that they are not designed for long periods of use. As for as the substrate goes, sure, aspen can be consumed, as can any substrate including the cat litter you are currently using. It's really just personal preference but most prefer aspen or cyprus mulch because it holds humidity well.
Hope this helps clarify a little bit and welcome to BP.net