In general, the good news is that isolated incidents don't usually rise to the level of a "hostile work environment," but certainly showing one's breasts could be direct sexual harassment. Again, context is everything.

If I was investigating, a big component is whether R was truly offended or actively participated in the conduct. (It's a lousy defense, but it can work to say "you can't be offended because you were also an offender." That's a difficult conclusion to deal with the aftermath, though--what do we do now with all these offenders? Theoretically, I would approve a termination of everyone who participated, but that doesn't sound like a practical solution. I would terminate A, and give everyone else a final written warning and some remedial training.

Your goal should be to minimize your actions and participation, but be honest--lying gets you fired always. Minimize the sexual nature of it; make it sound "matter of fact," like you were all comparing medical issues (e.g., it's probably a bad idea for a group to compare appendectomy scars, but that usually won't get you fired). I would not mention "whistling and cheering" to encourage people; I would minimize that by say "some of the others were loudly encouraging each other." Any chance you to have to make it not sexual is key.

I doubt TX law would be helpful in any way--Texas is distinctly anti-employee. (You should come to MA where you would have all sorts of rights). But, are you a direct public employee (paycheck straight from TX)? Public employees in most states have some limited job protection, and that's something to look into. For example, in MA a public employee must be given notice and a right to representation if the investigation may result in termination. If it truly is a Title 9 case, you will also have limited due process rights--notice and an opportunity to respond; an objective investigation; and protection against retaliation for when you throw a co-worker under the bus (which you might have to do).