Can this be installed, over an underlayment, on top of radiant flooring film mats? Would there be off-gassing of the rubber by the mat's room temperature heating at ~ 100F-120F?

Yes, our Pre-Cut Eco-Flec Rolls can be installed over radiant flooring film mats. Rubber naturally will off-gas. The heat won't make this any worse. If this is used as an underlay, the smell will disappear once you install your flooring over it. If you are sensitive to smells we recommend mopping the floor with a mild soap and water solution and then letting the flooring air dry. You may have to repeat this a few times to make the smell go