CTS-500 is indeed repackaged RSTS. RSTS itself is a timesharing system, you don't need to layer anything on top of it. The packaging included, if I remember correctly, some stuff aimed at business applications such as an indexed file system. (This predates the support of RMS-11 in RSTS, and RMS-11 basically eliminated the reason for having CTS-500.)
There were some PDP-11s with special paint jobs (blue rather than maroon) and bezels, and also I think lower cabinets, as part of the CTS-500 effort.
Duress was similar to Dec's DOS to the outside user. The motivation for writing the system was to take full advantage of the addressing modes of the 11/45, and may indeed be the only working system to use all three addressing modes. The system used K-space for the low- level kernel, device drivers and task management; S-space (Supervisor) for the disk file system; and U (user) for application code. Almost all other systems ignored S-space and put the entire OS in K-space. This implementation effectively doubled the amount of resident memory available for the OS.
The system was used for a phototypesetting system for the State of Wisconsin Legislature, and later was used for a variety of financial and newspaper typesetting and editing systems.
By V2.5, MPP was highly reliable. The customized Pascal compiler had some issues, but I did most of my development in MACRO-11. We built ROM-based high-speed trunked radio system controllers with it at GE, and found it very satisfactory.
The web page for TRIPOS is at http://www.cl.cam.ac.uk/users/mr/Tripos.html, but it does not mention the PDP-11 support. TRIPOS is written mostly in BCPL.