Thanks for the feedback!
If you hadn't already found it, it is possible to customize the keybindings from the Options screen in order to change the character selection keys.
Thanks. I did in the end just swap 1,2,3 with Shift-1, Shift-2, Shift-3. This works fine for me. I use Shift to build/act. I could recommend these as defaults for first time players.
Additional feedback! Sometimes one of my characters gets attacked off-screen while I am focusing on another one. I believe there is an indicator for that (pulsing green border?) but it would be great if it was more visible (perhaps a larger red pulse), and perhaps accompanied by a sound when it first occurs. Usually when I notice, it is too late!
Another thing that might help this is if fights lasted longer. For example, if all the actors do less damage, or all the actors have more health. That would give me time to respond when an attack happens. It would also mean less waiting between waves, because I would still be busy finishing off the previous wave! Obviously this would be a big change, but perhaps an interesting experiment.