It seems like the simple solution is to detect when there is more than one address that a reply could go to, and if so, on clicking reply have a sub-meny that a

It seems like the simple solution is to detect when there is more than one address that a reply could go to, and if so, on clicking reply have a sub-meny that asks whether you want to reply to all or one person (and maybe list recipients with the Sender at the top). You still get a single click to reply for single senders, but any ambiguity needs to be dealt with, but usually only with a quick additional click. I routinely experience this where I want to reply all in my work Gmail, but the default is reply to sender, and I end up having to re-send the email a few seconds later so everyone sees it (if I'm lucky and notice. Otherwise other people just never see what I intended them too).