My concern is that neither /category/ nor /c/ have semantic meaning for normal humans. They could be done away with entirely and humans would have both a shorter URL as well as a URL that didn’t treat them to codely “wearing of underpants on the outside” as the URL is read.
Is there any reason to not simply have the URL be discoursesite.com/categoryname/topic-title/ ? This would keep all of the words in the URL relevant to what a user might find on the page when they get there. In the case of topics and categories crashing into one another a simple appending of -number to the end of the topic-title could resolve that.
In addition to resulting in a URL structure that was entirely semantic and related to the content one might expect to find on the page it would help with (here is where I dodge rotten vegetables) search engines. Inserting the /t/ and /c/ between the most semantically meaningful part of the URL and the beginning of the URL encourages to think the forum cares a great deal about “t” or “c” and that these are the two most important things for all of our discourse-enabled sites.
I realize that URL structure things are always a massive pain in the butt and that you have recently closed this topic and settled on the /t/ and /c/ approach. But I would ask gently that it be reconsidered for some time in the future.