I like Nassarius vibex and Florida fighting conch for sand bed stirring. Nassarius stirs up a shallow sand bed well when they come out of their hiding place in sand to feed on left over fish food, and Florida fighting conch are known to eat diatom and algae off sand particles. They stay small and don't easily starve to death like a queen conch.
For glass surface, you might try a variety of algae glazing snails. Margarita is one of those snails that are touted as great algae eating snails by many vendors, but Dr. Ron Shimek, who has been studying snails for a very long time, recommends against it in this article called [url+
http://www.reefkeeping.com/issues/2004-05/rs/index.php]Grazing snails Part 1[/url].
As for snails in a sump, I don't keep one myself (at least intentionally) and I cannot really comment on this since I don't have any experience with it. Does anyone keep snails in sump? Without any light to grow algae in the sump, I suspect that there is nothing for them to eat there. I have a refugium next to my sump and I see some collonista on some of the rocks in my sump near the light of the refugium. I suspect that they came in with the rocks when I moved the rocks from the display to the sump.
I don't know of any "detritus" eating snails. I wish I know one so I can put it to good use.
If you are really bothered by the crud that accumulates in the first part of the sump, you could put a filter sock or pad there to capture the incoming crud. I never had that problem with my 20GL sump. My strong skimmer must have been doing a really good job.
I recall that you have a HOT magnum filter. Can you put a gravel cleaning attachment made (sold separately nowadays) for the filter and just vacuum clean the section periodically?
Tomoko