I've fixed the issue with the To address. The folk who originally made most use of this plugin didn't want the To to be editable, so that's how I set it up.
I've added an option so if you use the 'Specify in field' option for your address, you can now make it either readonly, editable, or hide it.
I've also fixed the issue that was stopping the "show subject" option, and when I get a minute I'll update that option so it's the same as the To field, with a "hide, show, readonly" set of options.
I've also fixed an issue with the message area, which was messing up the sizing in certain WYSIWYG editors, whereby it the width would be pegged to the width of your default message, so if the default was only a few words, the message editor was way to thin.
Those 'invalid argument' warning could only have arisen if the table details you gave returned no rows when we queried it. I've fixed that, so it won't pitch the warning, and will just render a "No selectable addresses found" message on the form.
Changes are in github.
-- hugh