why not 뭘?
He's doing an action, not acting upon something. If it was 뭘 the translation wouldn't be the same, it'd be weird, like: "You are doing to the what?"
When is the 지금 in the beginning and when in the middle of the sentence. The other example had the 지금 after the "what"
Though I'm not completely sure, I believe it can be pretty much anywhere in the sentence and it'd be correct
how come 뭐 has no particles, shouldn't it be 뭘?
Shouldn't 뭐 지금 해? be accepted too?
can't it be 지금은?
It would mark 지금 as the topic, I think it emphasizes "now" more than the action you are asking about, but i'm pretty sure you can
Is 뭘 하고 있어? a correct sentence?
Why not "지금 해 뭐?" ?
The sentence must end with a verb or a adjective.
Why is 너는 지금 뭐 해 wrong?
Is it not pretty common to put 지금 at the end too?
Pretty sure that's more of an afterthought while speaking, just like finishing a question suddenly with 오늘은.