This configuration worked up to now with no problem at all. Using transmission a swap partition is a must, otherwise when transferring large files you get out of memory errors with unpredictable results.
Do you think that the web UI gets confused trying to set-up the wrong partition? I Own a dsl-n66u on which asus allows to set a swap (don't kow if partition or file) directly from the Web UI. A feature they should add even to their routers.
Took me two nights of digging through the new USB handling code that Asus implemented in 376_2769, but I should have at least a working workaround now.
https://github.com/RMerl/asuswrt-merlin/commit/0843f3ed7243eb00d6852f468ca0dbdd55d01a64
Basically, Asus enumerates all partitions, but when trying to access the list, it only counts those that are mounted, so the two indexes don't match.