Is it okay to use a microwave in convection mode without the turntable?
I'm specifically answering your question "Is it okay to use a microwave in convection mode without the turntable?" -- Yes.

Your own answer points out the reality that if a pan is too large then the turn table will simply hit the sides of the microwave oven and then stop turning. There is nothing wrong with this. There is nothing wrong or damaging to have a metal tin touch the metal sides of your oven.

How was your baked items bottom? If you can sit your tin up so there is air flow under your tin, then you will improve the baking efficiency and it will bake more evenly.

CAUTION: If anyone else is reading this question and wondering about their own machine, then you really need to take caution about the "convention only" mode. You have to make sure there are NO microwaves in this mode or else your metal will conduct and you could have a serious explosion or break your microwave oven.