The President...shall have Power to grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment.U.S. Const art. II, Sec. 2, Cl. 1.
A Presidential pardon or reprieve should be the end of all legal action against Arpaio on this matter. Otherwise you have illegal double jeopardy.
What is going on here?
It looks like Arpaio is trying to find out if his conviction would have been overturned regardless of the pardon.
I’m pretty sure Arpaio could just walk away.