It is nice to see you have your concerns, but I can assure you none of these practices would have inflicted harm on the mantis. Freezing it would have numbed any feeling of pain, holding it down ensured it wouldn't hurt itself, the table I can assure you has done little harm to the thing (If you have ever been around tables they aren't naturally aggressive). Having it upside down is just a natural thing for a mantis (Paragraph 4 first line). It was all done for the sake of science and for a fast and painless job.

I do so believe that everything I have just stated is common knowledge and therefore I did not post any citations beside the information gathered in the article above.