This is an interesting use of the FBX SDK in Unreal, that’s for sure! I’m curious what cool benefits you get from using the FBX APIs to construct geometry in the editor versus creating mesh assets directly.
You are mostly clear on the legal side. This is because your editor customization is only useful to other Unreal Engine subscribers which is a valid use of the license. It is similar to distributing source to an editor plugin that you’ve created.
However you should not paste Epic’s FBX importer code to the public forums. Remember that access to these forums is not limited to UE4 subscribers, and the license only permits small “snippets” of UE4 code to be shared broadly. Instead, you should create a GitHub fork and link to your fork here, as only other subscribers will have access.
–Mike