Damage due to accident, abuse, neglect, misuse (including faulty installation, repair, or maintenance by anyone other than Apple or an Apple Authorized Service Provider), unauthorized modification, improper environment (including lack of proper temperature or humidity), unusual physical or electrical stress or interference, failure or fluctuation of electrical power, lightning, static electricity, fire, or acts of God;
You're fine, but any damage caused by the modification will not be covered. Note, however, that Apple may blame unrelated issues on your modification, and then you have to convince them otherwise.
No, not exactly. Authorized Apple Service Providers do not work for Apple, rather they are licensed by Apple to perform in-warranty and out-of-warranty repairs on Apple computers.
No, not exactly. Authorized Apple Service Providers do not work for Apple, rather they are licensed by Apple to perform in-warranty and out-of-warranty repairs on Apple computers.
Yes, you should be fine. However, if your machine does need service, it is sometimes just easier to put everything back (the way you bought it) - if that's possible with your modification.
For example, RAM is considered a user-installable upgrade, but even then it's easier to put that original RAM back in the machine before sending it in for repair. Just makes life easier.
One last thing to keep in mind. I came across this article from 2004, which states that Apple's warranty includes the phrase "This warranty does not apply...if the product has been modified without the written permission of Apple..." However, I don't see that clause in the current warranty.