If I chose a party do I have to vote for all of their candidates?
It depends on the election. In primary elections, you can only vote for candidates from the party to which you belong. In general elections, you are free to vote for anyone you like, regardless of their party affiliation.
Do absentee ballots even get counted?
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.