1. You can clear all the history, but you can’t clear only those files that don’t exist. I might think about adding a new command for this in the future.
2. You can uncheck “Strip Path for Files in Current Folder” in the History tab of Customize dialog box, but the path will still become shortened using “…” if the path length is more than a certain character long. If this is what you need, I will make the path name length adjustable.