Slide viewers are *no* good if they get hot. At best they'll only buckle the slides. At worst ...
Make yourself a platen mask of black paper/card (with the slit cut out at the bottom) to use as a guide under the light, and then fill the space with the slide mounts all for one huge scan. Come back in the evening to divide them up. Doing four using the plastic holder is a bit of a waste of time.
The other thing, which I've done with my dad's negatives, is only pull out the best for high res. All the lesser shots can be done at 1600 to make it quicker.