The slice offset slider certainly needs improvement. Most importantly, it needs a “snap to slice” mode (see for example discussion here How slice selection slider should work?), but it is a complex issue. For example, markups still need to be placed at arbitrary locations (not just at voxel centers), but then what would you display when the user wants to jump to a markup point location? A potential solution could be to add a “snap” button, which would jump to nearest slice and would probably override manual slice spacing. When slice position is set to a non-center voxel, or slice spacing is adjusted manually, or the slice view is rotated, then then “snap” mode could be automatically deactivated.
Disabling interpolation of grayscale images leads to many subtle problems. One of them is what you noted, but things even get worse when you have non-axis-aligned views, overlay model intersections, markups, etc. that I would not recommend anyone to use it.
Did you disable interpolation because you wanted to see voxel boundaries? For that, a proper solution would be to show a grid when you zoom in a lot - as it was done in Slicer-3.x. Unfortunately, this feature has not yet ported over to Slicer-4.x. It would not be hard to implement it, especially if you only need it for non-rotated views.
In your workflow, why did you adjusted the slice view spacing manually?