For the record: The days where u needed Low resolution HDR to light and high res to reflect have pretty much passed certainly with today’s offline brute force renderers and denoisers. So the high res reflection HDR is used to also light the scene.
You can tell unreal engine to override the reflection through material, you can also assign a custom probe. So in theory you should be able to cast diffuse light from skylight but have the assets reflect through a custom node.
I Usually keep the skylight as high res HDR and not have to worry about setting up this workflow unless it’s a specific case.
Side note: Low resolution HDR might benefit the lighting to make it less noisy in UE but I haven’t seen concrete results to show the effectiveness, it’s more of a settings issue. It’s worth noting though UE uses final gather type of method to distribute GI in baking this is similar to Mental ray, and in Mental ray the low res HDRI did have benefits for producing less of artifacts. You should probably do your own tests and see the results and if they are worth the extra effort.