I think a main part of your worries is that you are moving from an infant seat to a convertible, but are expecting the convertible to be more like an infant seat than it is. The infant bucket seats often fully support young infants including their heads and allow very little movement at all. The convertible seats which have infant inserts can duplicate this complete support. Once you take the infant insert out of the convertible seat (and it sounds like your son is too big for the infant insert--which at 7 months is to be expected) the children have more freedom to move their heads. At this age, they no longer NEED so much head support.
I also have a True Fit, and was greatly concerned the first time we put DS in it (he was about the same age as your son). It seemed to me like the gaping holes directly behind his head would bother him! I posted questions very similar to yours on a car seat forums website and got a lot of responses basically saying "Yes, that's correct. Don't worry about it". I tried using the 'little' head rest for a few days, but found it didn't stay in place very well, and so stopped worrying about it. (We had the seat installed closer to a 30 degree angle because DS was old enough for that--this is probably why the head thingy would sometimes try to slide down).
It's okay for your son to have more freedom of head movement now that he's older. One thing about the True Fit is that the upper portion is WIDE. This is kind of a drawback for younger children because when they fall asleep there is no readily available pillow for them to lean against. THe edge of the seat is REALLY far away from their head. Now at 2 years old our son is finally big enough that when he falls asleep his head can rest against the side of the True Fit and not look too uncomfortable. But honestly, even though it bothered me to look back and see his head practically leaning onto his own shoulder when he fell asleep--it never seemed to bother him. (He also gets into crazy sleep positions in his own crib, so I've just assumed that younger children are more flexible and less bothered by such things than we would be. If I slept for 30 minutes with head resting on my shoulder I'd have the worst neckache ever! But nothing like that ever bothered my son).
Another poster mentioned the Complete Air convertible seat. That one does have a lot of head support. We have a Complete Air in our backup car. If the freedom of head movement really bothers you, you might like the complete air. One negative thing I will say about it is that the straps twist ALL the time on our Complete Air.
How long do you have the 100% refund option? Could you try it for a bit and see how your son does? Otherwise, I would go to a store to physically look at different car seat options and see if any fit your needs--now that you know some more about what bothers you (though your son probably isn't bothered at all
Also, if you can you should get your carseat checked. It will make you feel better.
Finally, please don't get any after market car seat additions! These are just generally unsafe. If they are tested for safety (and not all are), they are probably not tested with your exact car seat configuration. These items can move and slide around and end up causing potentially dangerous situations. DS was about 6 months old before I realized after market car seat additions were no-nos, and so I sheepishly threw out all that kind of stuff that I had bought