JohnB
A " a full file verify" will nOT fix any issue with a 3rd party route unless it has modified any DTG/RSC DLC.
A verification of the game files does not remove anything outside the .ap file structure it can't because it doesn't know about them. The procedure merely overwrites (NOT deletes) any modified .ap file (and obviously any pre-.ap file it will overwrite any mod there.)
TS Steve on the official TS form states
. . . , when using the integrity verification check, Steam only checks the files that are actually part of the original upload and registered against the depot. It ignores all other files in the folder. So, unless the mod, EP, or other pack is directly changing one of the core files (and there are instances where people have physically altered values in the core blueprints), then only those are reverted.
However, the above only applies for products that sit within archived asset pack (.ap) formats. There are still a fair few depots (add-ons) where there is no asset pack archive and the files are in 'flat format' (i.e. not archived).
PeterH