You are correct that importing the character and the clothing as a single object will not allow you to remove the clothing. You should instead import them as two separate objects, then use the master pose component node to attach the clothing to the character.
To fit clothing to the character, I will export the scene to Blender via Diffeomorphic, transfer any shapekeys from the character mesh onto the clothing, then fix any clipping or other issues with the morphs by using the sculpting tools. After I do that, I export the character mesh + armature as one file, then the clothing + armature as another file. And when I import the clothing in UE, I select the same skeleton as the character mesh. Then attach using master pose component.