You need to get your Passat properly diagnose first.
It could be anything from faulty hardware or electronics to a simple re-calibration/re-set of the ACC module. A diagnostic report will give you that information to go forward.
Audi A8L adaptive cruise control system reset - YouTube
ACC calibration - Audi VW radar alignment DIY (C1103) - YouTube
Some videos to get you started.

To your questions.
Q1. The diagnostic device, be VCDS or something cheaper, is the first part of finding your issue. It will provide details on where your fault is and the codes related to that for you to research a path for repair.
Q2. Workshop manuals help navigate your way around the vehicle. ERWIN works by placing in your VIN and then downloading all the files which appear related to that VIN. The search function, unless you are familiar with using, can take some practice using.
Q3. Dedicated web sites, such as Ross Tech or forums, will provide you better instructions for coding than workshop manuals. I like to think that if you can see it then the manual will show where and what it is and how to get to it.
Q4. Most diagnostic devices will allow you the ability to reset warnings such as oil service.
Q5. This is a start ACC Delete using vcds, audi SQ5 - YouTube to assist in disabling ACC. You need to research further on forums or try ODB11 or Ross Tech's sites for guidance.