EULA License Grant Section 1a:
“Distribution to end users - You may Distribute Engine Code licensed to you under
Agreement (including as modified by you under the License) incorporated in
object code format as an inseparable part of a Product to end users who are subject to
an end user license agreement which explicitly disclaims any representations,
warranties, conditions, and liabilities related to the Licensed Technology. However,
the Product may not consist of or contain any Engine Tools (i.e., the Product may not
Distribute or statically or dynamically link against modules in either the Developer or
Editor source folders) nor provide functionality allowing the creation of standalone
products utilizing the Licensed Technology”
Emphasis Mine
You can write your own C++ API as long as it is devoid of UE4 types, and you can’t ship editor or tools code. Feel free to bundle a stand-alone FBX importer or whatever you need to allow importing meshes etc. You may have to provide your own mesh components to render them, I’m not sure if UStaticMeshComponent can be created dynamically but Epic does have an FDynamicMeshBuilder (or something to that effect).