Can I use these interchangeably?

(Someone's upset that they couldn't stop an accident, etc. in the past).

If I say,

How could you possibly have helped?
What could you possibly have done to help?

Question

Can I use these interchangeably?

(Someone's upset that they couldn't stop an accident, etc.  in the past).

If I say,

How could you possibly have helped?
What could you possibly have done to help?

Answer

Your first two sentences mean the same thing to me and are both correct. The sentence in your follow-up comment is slightly different in meaning but still correct and possible in the same situation.

Answer

And this,

Who could possibly have done anything?