If a gentlemen in this "hobby" was asked a question via email pertaining to a specific act a provider performed during the encounter, can his response be used as evidence in a court of law to incriminate the provider?

Generally, they would be hearsay and inadmissible in your state.  But there are a couple dozen exceptions to hearsay, and whether a court believes one depends a lot on the lawyers involved.

My guess is the bigger problem would be authentication:  prosecutor waves a stack of e-mails, and the very sensible question is, what are those, where did you get them, and show me some connection between them and the accusations against this defendant.

That said, don't be saying things, OK?  And especially don't believe that your simple assertion ("users agree that we are not soliciting prostitution") is binding on anybody, particularly a 3rd party govt agency.   These "money exchanged is not...etc" are the biggest flags and
"yeah right" signs a court can get.  Just makes you look like a sea lawyer.

Not a lawyer but talk like one....

My guess is that it will need corraboration

That's a difficult one to answer.  If it was a question, I'm not so sure it has much in the way of evidentiary value, unless it is a part of a larger conversation.

Also, was it the provider asking the question or a third party?  If it's a third party, it is heresay and can not come in unless the questioner is available to testify.

Yes, I do practice law.

No, it would be hearsay; to use anything, he would have to be a witness and be asked the question (assuming he was identified and subpoened); But if he gave an opposite answer in testimony, his response could then be introduced in an effort to impeach his actual testimony.

eom