Nice job on the write-up, it reads like a book!
That is a unique printed carriage design for the extrusions. One thing that caught my eye, It looks like the lever arm for the belt connection is fairly far away from the rail (15mm or so)? Given the friction of sliding parts, I would do everything I could to try to minimize the belt to rail distance. This will help with lowering friction, as well as increasing precision (less rocking).
I looks like the carriage is printed in its orientation while the mount is printed laying flat, but if you could combine them somehow into one print that would give the best result. Or if you moved the mounting holes to the sides of the carriage, that would save 5mm at least.