I was using the official UE Viewer version that can be obtained from
this page, and it's the one I said that works (which is version 1589). However, the material viewer version I used was outdated and didn't show the textures properly. The new version (uploaded yesterday) does work, so that's a good addition.
I have updated my post to reflect this information. The build 1579 (the one you specified
here (direct download
link) does work for both showing the textures correctly and loading/exporting the textures. Thank you! Version 4.21 is still needed to select to browse the materials. Otherwise, the viewer will crash.