OK, I have moved forward on this. My current approach is a bit of a change to what we had before. Not toomajor, but I hope a lot more usable!
The first key change is that when you go to the Messages tab, you will enter with a full list of all your inbox messages in chronological order just as you see when you switch to the Sent view. This will be the default view, you can of course use the country filters as desiredin either Inbox or Sent views.
The next kay change is that instead of messages you are writing appearing in the same display (the inbox/sentbox whcichever you are looking at) they will open in a new window. so for example if you compose a message / click to reply to a message / click to forward a message, a new window opens for you to create the message. In addition, for this new windo, the folders etc side bar disappears - you simply get to choose who to send it to and have the text area to write your message / response etc. This makes the view a lot more helpful on mobile screens since when you are writing a message the filters are rather superfluous.
The other advantage which I hope may be useful is this means you can open a window to create a message and keep the window open while you do other things. So for example you may want to write a message to Germany, but as you write it you may want to skip back to the inbox to look at other messages you received from other players to decide what you want to say. When the new message is Submitted or saved as a draft, the window closes.
The other change may be a little annoying to people on browsers or even ipads, but will hopefully be useful for phone users. The current column for time sent (real and gametime) will disappear but instead the info will be on the line below the title to the right (in grey italics). The info is all still there, and it makes it a lot easier to deal with messages on mobiles because you don't have to try to squeeze the info into a little column on its own, but what it does do of course is it means there is essentially a spacer between each message title. Actually, it almost seems clear to read that way with what looks like a blank line under each message title in my opinion - sometimes when they are all crowded on top of one another it can be hard to distinguish! But maybe that is just me rationalizing
Apart from that, some of the text is changing and I think I will make some of the options in the sidebar of the inbox/sent views into buttons. So for example makybe keep the country filters as tagged country names as at the moment, but make Compose Message a button, and maybe the toggle between inbox and sent views.
One thing I wondered about but which might be a) too confusing ans b) possible a bit risky is to allow multiple message creation/reply windows open at any time. So you could have 3 different messages on the go. Not sure about that though - although at the moment if you create a new message you will lose the window already there (I will put a confirmation box in though if we stick with the one).