It's debatable. In the future, specify exactly what file formats you will provide, the price and the estimated delivery date. You may be able to charge a little more for illustrator files since fewer people can provide them.
Some clients will specifically request the illustrator files, and frankly, they may not be capable of modifying them effectively, but sometimes you just have to let go of the work after it's done. If you are making a living providing commercial art, it is a waste of time to obsess about how someone else may mangle one piece of art that does not even have your name attached to it. Pretty often, they just want to be able to re-size without losing image quality.
No, you are not. Unless that was part of the deal.
Give the client a finished work they can display or use, not something they can edit themselves. Not even with billboards and stuff- they get an image of the image they can use, not the actual file. That belongs to you. If they need it worked on or edited, they can either buy the file off of you for a premium or have you adjust it yourself.
I think it depends on the project if was something say like a Billboard. I think you are required to give out the illustrator file, either that charge more for illustrator file. I'm not a professional just guessing. If you feel insecure them editing it I say charge more for the work. That's what concept artist do they draw and sell their 'service' and forget about it. The client has to option finish up the drawing from another artist, draw more concept about it from another artist, or continue to finish up/build up that drawing from you as well and what not. It's all contract based, you write it up in your contract about editing. IMO I don't them editing it they only charging you for the service.