Is it insulting to call a full professor Dr. Firstname Lastname in a non-academic event program?

For most people, insult and offense comes more from lack of care and respect than from mistakes. I would thus recommend the following procedure:

1. Prepare a draft, making your best guess for each speaker based on their web presence.
2. Send out the draft agenda to all speakers, saying that this is the draft and you'd like corrections in case you have made mistakes in how anybody is listed. (You probably want to run the schedule by them anyway for other purposes as well.)
3. Make any corrections requested by the speakers.

This way, if you get anything wrong, you're doing it in private, acknowledging possible errors, and giving them a chance to correct you in private as well. It will be a rarely sensitive person or an unusually significant mistake that will cause offense in this way.

This strikes me as more an English usage question than an academia question.

In American English, "Doctor" is a personal honorific, whereas "Professor" is merely the name of a job. "Dr. Firstname Lastname" is the correct formal introduction in AmEng for anyone possessing a doctorate-level degree (PhD, MD, etc); the academic position they hold (if any) is irrelevant. As a native speaker of this form of English, "Prof. Firstname Lastname" sounds stilted to me and "Prof. Firstname Lastname, Professor of ..." sounds redundant.

However, as pointed out in the comments, in British English, "Professor" is considered a personal honorific, so some of your speakers may prefer to be introduced as "Prof. Firstname Lastname". The only way to be sure you don't irritate anyone is, as jakebeal suggests, to draft the program and then run it by everyone and ask for corrections.

In all forms of English (that I know of), honorifics are never stacked. "Prof. Dr." will read as a non-native speaker error. (Multiple postnominals - "Dr. Snooty McSnootface, M.D., Ph.D." - are not unheard of, but they read as egotistical. I would not put any postnominals on anyone's name in this context unless they themselves insisted on it.)

I would personally omit titles in such a place at all, and just list the profession, which shows clearly that they are professors:

Topic Title - John Doe, professor of nothing at University of Neverland.

If you do the same with everybody, you're fine: you can't insult people this way. I would say that the only people who really need a proper salutation always are Her Majesty and people of similar kind. With the others, it's just a matter of consistency.

Note that this may be field-dependent. As usually, in math and related field, people are more relaxed, in other field, this may be different.