It shouldn't be hard to get it "in writing" with a simple web search. It's the law. They can't force you to use them for service, have to perform recall services for free, can't stop you from doing your own maintenance, and can't void your warranty except in cases where your negligence caused the problem. And in those cases, it would be for the relevant component, not the entire car.
If something is leaking, and they can show that it was leaking at the time you were due for annual service but you failed to do it, and the lack of service caused damage, that would be on you. But it wouldn't mean that if your screen broke or your headlights stopped working, it wouldn't be covered.
Also, most if not all of the parts needed wouldn't be relevant to warranty. If you keep your car garaged and drive for a few thousand miles per year, your wiper blades will probably be like new, and your air filter might not be dirty. If you don't change the batteries in your sun visor, the worst case might be having to change them later, if you even use the battery. Just remember to read the instructions and warnings so you know not to eat the batteries. I assume that people who don't read that assume that batteries are snack food, or they wouldn't have put it in the manual.
Regardless, annual service is cheap enough, but don't worry about losing your warranty as a result of getting your service a few month's late.
