CB Auto Actions has no means of keeping track of any of that. Whether the action executes or not is entirely on the Conditions you have configured. You'd need to keep track of if they were emailed then condition for that in your email action. CB Auto Actions is not a scheduler or mailer of any sort. It simply executes actions on various triggers based off the supplied conditions.

One way you can have what you're wanting is to create a checkbox field that you toggle to a value of 1 when they are emailed. Then in your email action condition for that field to not be equal to 1. To do this you'd have 2 auto actions (1 email and 1 field) that have the same trigger, etc.. so they both fire (1 after another with field change being last).

Correct, the message does not support HTML. You have to use a Link with the Type set to Custom to insert custom HTML. The message is freely editable by the user so it only permits the same content a user themselves can supply. Users can not insert Links of Custom type so you're free to add whatever you like to those.