List mode vs. Preview mode is a personal preference but I believe List mode would provide the functionality that you are looking for.  One of the ways to make List mode very  useable is to keep multiple folders open in appropriately sized windows on the Pegasus Mail desktop.  At a minimum keep the New Mail folder and the Folder list open but perhaps the Main folder as well.

Currently when I delete an email that is displayed in the preview plane, the program drops down to the next message on the list and insists on getting that next message displayed before the program will accept other input. I would like two things actually.

 I would like the program to be open to new input even while it is attempting to display something in a preview plane. If that means that PM needs to be able to use more of my computer's resources I would be fine with that. I tend to become irrationally irascible when the fact that I chose to do A means that I must do B or cannot do C.

 I would like the program NOT to display a message in the preview plane unless I click on that message. It does occur to me however that this may be a problem in Windows not in PM.