Many image editing programs, such as GIMP, Photoshop and Affinity Photo, offer batch processing options. Simply set up a batch process to resize the 8k textures into something smaller, such as 50% resolution for 4k maps, or 25% resolution for 2k maps, and the program should automate the process for you. Then swap out the files in the Blender scene for the new, smaller ones.

Yes! The maps are TIFF files, and are contained in an easily accessible folder named "textures". You should be able to use them in virtually any render engine.