Yes and No. In Iowa, you must be a member of a political party (currently: Republican and Democratic) to vote in a primary election. However, anyone* can change parties on Election Day and vote. For independents to vote they must declare a political party.

*Precinct election officials may not change political parties within 30 days of an election.

Yes. There is a myth that absentee ballots only get counted if the election is close, however, this is not true. Under Iowa law, every ballot must be counted.