Thanks for the kind words Chris!
Atlas had an (IMHO) very good runtime - I believe on that side it was stable (and even did some neat for the time stuff like polysoup collision).
But as noted by Chris the workflow was weak to nonexistent. I did some initial work on roundtripping chunk geometry into 3d editors, which unfortunately never got to a production ready state.
I based it largely off of http://tulrich.com/geekstuff/chunklod.html and would suggest that for your scenario this might be a great direction to go. In the olden days we had to really be aggressive about polycount so decimation was very important. Nowadays it's not nearly as important and may be entirely optional if you have a reasonably beefy GPU. However you would want to merge meshes if you have a long view distance to avoid blowing out your draw call budget.
I explored some techniques from http://number-none.com/product/Unified% ... 0Part%201/ for LOD but upon checking the code I don't think I ever got them into production.
Anyway, if you have already textured chunks, I would suggest simply streaming them into Torque and keeping an LRU cache. If you have LOD then that's even better, you can swap to it on the fly. If not you probably need to merge up by directly combining meshes. The 65k vertex limit will probably be annoying in this context. Or maybe your view distance is short enough you don't have to worry about that.
Let me know if I can address anything else to help.