Please let me know your valuable inputs on this.

My family is in my home country now. Our H1 and H4 are stamped upto June-2018.
I already submitted H1B extension petition. By mistake we submitted I-539 also for H4 extension also.
My questions are:
1. Can my family travel to US in their existing H4 (valid upto June-2018) in April-2018 and go back to home country in May-2018? Will they face any issue in PoE due to visa expiry in Jun-2018?
2. How can I withdraw the I-539, since I am not supposed to submit it while dependents are outside US. Can my dependents send a letter to USCIS signed by them? Do I need to sign that?

Thanks in advance.