Roundcube would probably work really well if you could double click in MobileSafari.
As it happens, I ran into that issue.
I worked around it by increasing the double-click time in RoundCube by editing the file program/js/app.js. The double-click time is set by the line "this.dblclick_time = 500;". I changed it to 800, and now if I'm careful I can double-click to open the message without zooming in Mobile Safari.
A better solution would be to add a "view message" button to the button bar. This would also help users who simply have difficulty double-clicking. It's probably a very simple patch to write, but I haven't gotten into the source code at all yet...
Note that it's also a bit tricky to scroll the message pane in Mobile Safari, as dragging with one finger tries to scroll the entire browser window. However, if you drag with two fingers, the message pane will scroll and the browser window will stay put.
Best,
Franzzup