Punctuation of an exclamative question

According to the Interrobang wikipedia page, this symbol [‽] has been created to convey the meaning of your question. Using it you ask "a question in an excited manner, express excitement or disbelief in the form of a question, or ask a rhetorical question."

But, always in that page, you can see it's a nonstandard symbol, so like it says, "in standard English, the same inflection is usually notated by ending a sentence with first a question mark and then an exclamation mark."

Now, it's also true that in an informal context, most people won't really care about it, but if you must choose a "line", it's that one.
Concerning formal writing, I'd suggest to avoid any of these and focus on the wording to convey the "exclamative" tone of the question.

The question mark and exclamation point should not be used together as in your examples 1 and 2.

The simple solution here would be to use the exclamation point only, treating the sentence as an indirection question:

What are you doing?

Yet, there are other ways of indicating emphasis. Consider italicizing the key word (from your perspective):

What are you doing?

Or, you could employ additional text for clarification, e.g.:

"What are you doing?" she shouted at him.