The short answer is probably Yes to all of your questions, but you will need to speak to a lawyer about these things after he is aquitted. Until that occurs, the first order of business is to get him defended. He should not be trying to explain himself, he should exercise his right to remain silent, and let his lawyer address the present legal issues. Good luck.
DISCLAIMER Since I do not practice law in your State, this answer is provided solely for informational purposes only, for you to use as a starting point when speaking directly with a lawyer in your State. This answer does not constitute legal advice, create an attorney-client relationship, or constitute attorney advertising. I urge you to immediately contact an experienced criminal defense lawyer admitted to practice law in your State before you make any decisions about this case.
Step one is the successful defense of any charges filed against him. Step two will be convincing law enforcement and the State Attorney to pursue charges against her, IF the facts are even appropriate to do so. You will be fighting the CAN we file charges issue as well as SHOULD we file charges discretionary issue. As someone who spet 6 years prosecuting sex crimes/child abuse, it is a very rare move to turn around and prosecute a victim/accuser/complainant, whatever you want to call the girl. Also, please don't underestimate how little physical contact it requires to sustain those charges. The lewd molestation statute covers not only the touching of genitals, buttocks, breasts, but also the clothing covering those body parts. So long as suitable context can be used to prove that the touchig was done with a lewd/sexual intent, there is a cerain amount of exposure to being found delinquent/guilty that I wouldn't just call unprovable because the bus video (never seen anything but crummy black and white school bus videos) doesn't show whatever happened in a close-up high definition shot.
Is he serving jail time because he entered a plea to the charges? If so, his case is essentially over if his plea was entered with his full knowledge. As for the filing a false report, it will be up to the police and state attorney's office to decide whether or not to file formal charges against her. The state attorney can pursue charges if they wish, but if your cousin entered a plea of guilty or no contest to the charges the victim lied about, the chances decrease.
If your cousin has not entered a plea, a criminal defense lawyer can get all the evidence in the case, also called discovery. The prosecutor will have to turn over all evidence he or she has, even if it is exculpatory. That includes any video evidence.
You should contact a criminal lawyer in your area to discuss his case further.
This answer is for informational purposes only. This answer does not constitute legal advice, create an attorney-client relationship, or constitute attorney advertising.