I cannot push my head back against the headrest on Incredicoaster. It is super uncomfortable for me and it gives me a headache. I will have it against the headrest during the initial launch, but immediately pull away about an inch or two as soon as the train is moving. Otherwise, my head gets banged around on the sides by the shoulder restraint. I'm short, though. For the rest of the ride, including the loop, I lean forward a bit. It's far more comfortable. This is a very individual preference.
That said, the Incredicoaster is very enjoyable and QUITE mild when you sit in the first 2 cars of the train. I always suggest nervous riders ask for the front row. It helps a lot to be able to see what's coming and the front is the least jarring experience. The Incredicoaster is over 20 years old now and is a bit of a rough ride in certain trains, especially at the bottoms of hills and in the loop. You will feel some significant vibrations during these times, especially in the back of the train.
As for the loop, when you are on a coaster loop you don't FEEL like you are upside down. You just feel heavier. All you see is the track coming towards you. It's actually the least scary part of the ride. As far as inversions go, IC is SUPER mild. Newer coasters like Velicicoaster have high speed barrel rolls and such that really do disorient you.