What utensil (material) to use in Convection + Grill mode of Microwave Oven?
Preheat should be done with an empty oven, afaik. And indeed, a grill isn't nice to plastic; and I avoid plastic also when using the convection mode (alone or with microwave): where the plastic touches the food, the temperature will be limited by the water in the food, but if the plastic is not in contact with the food, it'll go to the temperature of the oven.

Convection heating is through air circulation, so IR plays only a very limited role there. Borosilicate glass is however a poor conductor of heat, compared to metal. That means that a borosilicate tray will show more browning on top when the interior is cooked. But it is perfectly usuable in a convection/microwave oven. And you can always give it a few minutes with just the grill to brown the top a bit more.

As for the use of metal in yor microwave oven, check the manual. It might be possible with limitations.