Sorry to hear you are still suffering so badly with your health, and I hope there is a brighter light on the horizon and you are pulling through OK. All of our thoughts are with you, and we certainly appreciate everything that you do in the face of such obstacles. Keep Moving Forward mate, and the very best of luck to you.
My personal everyday device is a Galaxy Note 3, I get 9 message list items per screen (in portrait) at the smallest font setting.
What about you? The "9" is with the default wide padding top/bottom inside each message list item. Turning it off should fit even more -- this is in app settings -> message list -> "use slim padding".
I use slim padding, and get five and a half items on the message list screen (like you, on a Note 3). I have the Line-1 and Line-2 fonts set to Small, and Preview text to Extra-small. I did have AquaMail set to show three preview lines, but I've cut that down to two to even get the 5.5 items per page - but I'd prefer three lines of smaller-still text.
Andre