Let's cut to the chase:
1) Concur with deanej... A DLL file in CustomAssets isn't loaded by the game, so putting a DLL there is the same as not having it at all.
2) What is the PURPOSE of the custom DLL you are trying so hard to use? What changes are in the custom DLL that you want/need?
3) Most mods that use a custom DLL use it for one or more significant changes to gameplay... either more then 18 civs in a game (40 or 50 civs), a significant rule change like adding Dales' Combat Mod(s), or some other basic change to gameplay... if all you seek is some snazzier graphics and/or a few new units or stat changes, all of that is accomplished without a new DLL... you only need a custom DLL if you are changing gameplay in some significant way outside of new units or graphics.
4) Some mods that come with custom DLLs (such as my own) will work just fine without the custom DLL... you just get the mod without whatever rules changing was in there... in my own mod, for instance, the most important DLL change was adding support for 40-civs in a single scenario... you can play my mod without the DLL and it looks just the same, you just don't get support for 40-civs.
Having said the above... once again... what is your purpose for a custom DLL? Also... I believe you have v3.17 installed, but I have a very strong belief you are trying to use a custom DLL that is designed for v3.13... that simply won't work... custom DLLs for 3.13 just don't work with v3.17.