If I wanted to use a different fork of Dosbox to play those (like Dosbox-X or Dosbox Staging), what specific files from the UW1 and UW2 GOG installers would I need? I also notice there are .gog CD image files, so what are those image files used for?

it'd probably be easier to switch out the DOSBox within the gog installation of the games than vice-versa.

the original games were meant to be played with the CD in the CD-ROM drive, they access data from the CD during play, that's what those image files are for. among other things, the main executable of each game is run from the (real or virtual) CD.

also, consider playing the games with my patches:
https://github.com/JohnGlassmyer/UltimaHacks