I like the current way of opening it next to the pane that it was opened from.
Reusing the pane globally so that it opens more quickly would be an improvement.
How about once it is loaded, when it is closed, just hide it and when opened again, reuse it instead of loading it again?
Sometimes I find it would be nice to keep a message from my inbox open at the same time I keep a message from my outline open. Perhaps you could create one message pane attached to a source pane when a message is opened and keep that pane hidden when closed rather than destroying and reloading with the next message opened … In other words, keep up to one attached messasge pane per source pane. This isn’t super critical if having more than one message pane is challenging.
Regardless of the solution, it would be extremely helpful if the message that is currently shown is highlighted in some way in the source pane, whether it be the Inbox or outline, so it is easy to see in the source pane which message is open. This is useful when you want to act on the open message so it is easy to quickly identify and grab/drag or select and archive/delete.