If I want to use that very same plugin: must I download the master branch 4.18 from Github and copy/paste the code to that version and compile the whole engine? Or can I somehow rebuild it just using the binary version of the launcher?
You can place the plugin the folder /Myproject/Plugins on your outdated (v4.12.5). if it doesn’t exist, create the folder Plugins after that you can port it to a newer version and than you compile everything again from VS. i did that with a plugin from kinect 360 and it worked fine. I think you can put the outdated plugin on your actual project without opening it in an older version. As long as you place the source code on the /Myproject/Plugins folder and than compile it, you should be ok.
Aside of what ulisses177 said about project plugins, 4.12 is quite far away from 4.18, there possibility that you will need to fix some things on your own due to API changes, best way to do so update the plug in version by version and do all fixes that deprecation warnings suggests on each version. Because if you jump imdietly to 4.18 you will miss old deprecation messages that already got removed and you might have problem figure out how to fix them.
you need to build it in VS, it will build together with game project. Ah yes you need to make C++ project to make it work, you could also try generating project files by right clicking uproject file, i never tried that with just having preoject plugin so not sure if this gonna work