I guess the basic reason why you can’t delete voices is because that is how the software is designed. Maybe one of the development team will chip in to say why they made that decision.
Voices in Dorico are quite different from in Sibelius or Finale. In Sibelius you have a fixed number of voices per staff (4) which always “exist” in some sense of the word. They have fixed stem directions, and a set of (somewhat arbitrary) rules about collision avoidance (or not) between overlapping stems in different voices, etc.
In Dorico there is no limit to the number of voices, and voices are actually unique across all the staves of a multi-staff instrument, even though they are labelled (“up/downstem voice 1/2/3” etc) for each staff separately. Collisions are resolved based on the actual music on the staff, not by a fixed set of rules. There is no equivalent in Dorico of using voices 1 and 4 (but never using 2 and 3) in Sibelius to bypass some of the collision detection, for example. If you want to change how Dorico arranges the voices, either select the options you want in Engraving Options, or as a last resort you can assign the “voice column index” to each note in Engrave mode to specify exactly how the notes are stacked vertically.
Obviously it doesn’t make sense to “delete a voice” if there are notes or explicit rests in it. If a voice has no music in it, it is automatically removed when you close and reopen the project.
If you want to interchange the contents of two voices, select both sets of notes and do Voices / Swap Voice Contents.
For the unwanted rests, since you haven’t shown us a picture with voice colours we can’t guess why they are there, so we can’t tell you how to get rid of them! There are several possibilities, but trying to list all of them would probably just add to the confusion rather than solving the problem.
“Starting again” is quite a good learning strategy - if you aren’t sure how something works, create a new project and mess around, without the worry of trying to undo your mistakes, and then work on your real project.