I think there are two differences between truthfulness and honesty. The first difference is that a person can be honest but sometimes he or she can lie. The truthful person is a person who always tell truth. The second difference is that a honestly doesn't mean telling truth. A honest person can change the truth. But the truthful person always in any situation tell the truth.

Yes. Honesty is a personal quality. e.g. 'Thank you. I appreciate your honesty.' Truthfulness refers to a fact or statement. e.g. Everyone doubted the truthfulness of the evidence.