Myself here using the latest ue5 version from the launcher i have no such issues.
No problem right clicking and converting 20-30 images to virtual textures.
Might be worth to click on the dropdown on the engine version on the launcher and verify.
Perhaps some files are missing or corrupt and verifying may solve some problems.
And for your other question, Streaming virtual textures( so the non runtime variant)
are cooked and loaded from disk, then the gpu determines which tiles are visible on screen.
And loads the required tiles into the GPU memory cache! 
I suggest looking at the virtual texturing documentation, its very helpfull.