Not really a pro like people on this forum, but if I may offer my two cents regarding "safety" ...
Rough "cutting" edges are definitely a no, but if grinded down to something smoother, why not. As for the spikes, guess it depends on the size ... and how well they hold together. Snake is like a tank, he won't stop climbing them so they have to hold together very well.
As for being cleanable, acrylic colors + some water-based enamel (atested for being used with drinkable water/kid toys etc) should do the trick, I think it wouldn't be impossible to even put the enamel (lacquer?) directly on the polystyrene, but gotta count with bigger amount needed as it might soak some inside and then venting off ...
As for assembling together, not sure if paper-glue would be enough, but if not, you can try to usesome build-glue ... not sure how it's called in USA, here you can add it to a grout to enhance it's properties. I used it successfully on cardboard before and hell it holds together so well you rather tear the cardboard apart than disturb the connection.
Lastly, regarding the roughness ... I have grout covered shelf in my viv to offer something to rub onto during shed (one of the surfaces) and I made "spikes" on it with a brush while it was still moldable ... sorta looks like that nail-bed now, but not rough enough you would scrape your skin or something while gliding over with bare palm. Snake doesn't seem to have problem with it though
In general, try to make things "unbreakable" but still I think the risk of snake "eating" some pieces is practically nonexistent, as long as you won't feed in a viv while leaving those seemingly possibly torn off parts laying around.
Regarding substrate, as I don't have access to aspen, so far I'm using a pure, organic peat used for flowers. But without additives and chemical stuffs to enhance plant grow, just plain soil. It keeps humidity very well just finding a poo is a challenge, especially with small snake
Plus is that it's naturally "acidic" so any molds or other nasties will not start growing, cause they don't like acidic environment to begin with. No baking or sterilizing, it's a popular bedding here in CZ cause of lack of aspen.
I'm sorry for my english if something is hard to understand, it's not my native language, but I'm trying
Hope this helps!!