It won't hurt him to ingest a few aspen shavings. Their bodies can easily handle stuff like that. In the wild, they must accidentally ingest all sorts of dirt and leaves and twigs and grasses and junk.

Eating it backwards isn't going to hurt him either...it'll just take longer. Worst case scenario would be that he'd get frustrated and just give up and abandon the meal. Wouldn't hurt him.

I've had an exploding rodent...it's disgusting beyond measure. My snake ate it anyway...but then I had to scrub guts off the water bowl, which seemed to have cemented onto the surface in a matter of minutes!