An appraiser peer mentioned that I'm risking a potential sanction if I testify as an expert witness "without proper experience."
Nonsense.
As long as you're not testifying about something that you're not qualified to appraise, of course you can testify.
You are a professional appraiser, licensed by the state, and you routinely receive compensation for appraising real estate. You are an expert.
Now, if they want you to testify about something that you've not had experience appraising or is outside of your license qualification, that's a different story.
I prefer to not see any other appraisal. That way I can honestly say "I haven't seen the other report and cannot comment on its contents. I'm here to testify about my report." Otherwise, you can get into the quagmire of whether or not you performed a complete review, and by the way, where is your workfile for the review, did you personally inspect the other comps, etc.
Its a civil trial. From what I've seen, the best liar will win. Chances are, if the two reports are not far apart, they'll settle for the midpoint between the two. I sat in a courtroom for four hours and just before I was called to testify, the attorneys got together and averaged the two and sent me home. And I was paid well.