Where are you going to college in the fall? Any chance wherever you are going is test optional? If so they might not care, and voiding the 31 ACT would be a non-issue.
I might choose #3, take your IB tests and then reevaluate. Then, if your college requires a higher score than the 21, you can take it again.
There are several threads on this issue, I think this is the main one: http://talk.collegeconfidential.com/sat-act-tests-test-preparation/1987200-act-testing-wrongly-accusing-cheating-2017-p1.html