Overlapping slices are not possible at the moment. A slice simply begins at a given slice marker, and then its length/duration is determined by the position of the next slice marker (if any are present).
(Technically, you can change the slice playing behaviour so that slices will continue playing past their end point until manually stopped, but I doubt this is what you wanted?)
When working with a sliced instrument, the slice markers can only be viewed/edited from the instrument’s first sample slot. From there, you can easily adjust the markers, zoom in/out of the waveform for more precise editing, use the various snapping options, and so on.
It usually helps to disable the “auto-select sample” feature. This will prevent the waveform editor from automatically jumping to each slice as you play it, and instead allow you to focus on the main waveform and make the necessary adjustments. You can find this toggle switch in the upper left corner of the sampler tab.