The other big thing about the arrow keys is that up directly above down, not above and slightly to the right. The mouse isn't really used for anything other than dragging items though (and for that I usually take my hands off the keyboard), so I'd say leave it at E. The other option (which is appearing better and better the more I think about it) would be to entirely replace E with left-click. This would:
1. Reduce the confusion of dragging objects to diamonds with left click, while interaction is done with E. Instead, all interaction would be done with left-click, creating more internal consistency within the controls
2. Allow the player to select diamonds they are in range of without having to move toward them just to avoid instead selecting another diamond that is closer.
I probably botched that phrasing so here is an example, in which "c" is Curly and "d" and "D" are diamonds:
d c D
In this example, even though both diamonds are in range, Curly must move towards d in order not to accidentally select D. However, if selection were based on clicking, The player could simply click on d without having to move. The player would still be able to select D by clicking it.
3. Allow righties to have their left hand on wasd and their right hand on their mouse (or trackpad), while lefties could have their left hand on their mouse (or trackpad) and their right hand on the arrow keys.
The one problem I can think of with this approach is that space would be relatively unreachable. A possible solution would be to also remove space and have jumping be done by up arrow. Another solution would be to have jumping also be rebound to left-click, but this would throw a lot of people off (myself included).
For me at least though (I just tried this hand layout), my right hand on the arrow keys, my left thumb on space, and my left middle finger on E was pretty comfortable. In short, I honestly think that just allowing arrow keys to be used to move and not rebinding anything else would be fine, easy and simple to use. Take the rest of this comment with a grain of salt.