I have noticed on my new system that when I use an extra display it switches from integrated graphics into the more energy hogging extreme graphics. I don't see any way around this other than using the correct wattage adapter and plugging in during use.
Now I will say other than size I do like the Apple adapters there was some big chunkers at one point and until they made the cords detachable I replaced mine every time they frayed with real copper cabling.
So I would say the solution would be to plug-in when you can because you're going to put cycles on that non-replaceable battery and definitely for presentations unfortunately. The good news is there's most likely an outlet at the presentation station. Another possible option would be to get the higher power wattage adapter to help with that you're using the more powerful discrete graphics processor.