This is intentional, since normally after one executes a plot command they'd prefer to see the generated plot immediately. This, of course, is not true in your workflow.
This seems unintentional -- any chance you would be able to provide a reproducible example?
Would you mind filing this as a feature request at https://github.com/rstudio/rstudio/issues?
Similarly, a feature request here would be appreciated.
We are thinking about what it would take to move away from the current quadrant-based layout used by RStudio, to allow users more room in customizing and re-organizing the panes used by RStudio (as you are trying to do). I can't promise any timeline but it's high on our list of things to tackle next.