What are some options for this late, substance-less answer?

If a default was not entered prior to the answer, there is no real relief other than your ability to refer to the untimliness in future issues. If the defendant continues to be dilatory, you have the right to seek sanctions. However, courts are reluctant to award sanctions without a clear shwing of continious disrespect to the rules and the courts power. The rules of procedure require the same standard of pleading for defenses as is required for claims. Each defense must allege the legal elements and be supported by ultimate facts. A motion to strike the defenses tests the sufficiency of the defenses. You may consider sending a request for admissions, which requires an admission or a denial of the requested facts and If you prove a fact which a party has denied you may be entitled to an award of fees incurred to prove the denied fact. Most important in litigation, as the plaintiff, is to keep your ultimate goal in mind and not let the defense take you off track.

There are several discovery vehicles in the federal rules to sort through standard denials, and pre-emptive affirmative defenses. Raising them is not necessarily improper. Litigation is an adversarial process. The federal discovery rules are tricky for a lay person. Don't navigate them, alone.