Is it okay to use !? for emphasis in dialogues in a screenplay? I understand that it cannot be used in the ACTION or the PARENTHETIC, but I assume that the rules are a lot less stringent when it comes to DIALOGUE.
The use of an Interrobang is perfectly acceptable language usage, and there is no technical reason why it can't be used.
It would be difficult to assume that all of the readers of the screenplay would understand what the intended meaning is, so you may be better conveying that in a more explicit way. (which is a shame, because I do rather love the interrobang!)
Using !? is ok, when your reader knows what that mean in "words and expressions". If you are making a comics, it will be understand well, because you can catch somebody's face and its expression during the situation.
If you want to be a good storyteller, you need to explain the situation as well as face expression to your reader.
Using ? and ! separately on its own its common knowledge, !? is not.
But this is personal opinion.