#1. of course can adjust the price on your models at any time and looking at your models, you should definitely do so. Setting $10 price for detailed and well done car model is a crime.
#2. if 3d model is entirely made by you, then yes, you can sell it, but you have to sell it under editorial license if at least small part of it depicts branded/copyrighted thing.
#3. it's up to you, but i wouldn't bother. No serious customer will ever look at unfinished model and you will only be targetting occasional hobbyists. Although judging by your models' prices, you're not aiming anywhere higher.
#4. again, it's up to you, but if customer not asking to fix something that is broken, or not working how it should and instead requests to add additional details to the model, or modify it in some way, then you don't have an obligation to comply to his request and if you will, then you should charge for your services. Customer is right only when he's right.
