a) You have to agree, but if you don't they are free to reduce you to 0% with sufficient notice
b) If you agree, the notice period still applies. This was pointed out to me by the ALK.
Case in point, they wanted to reduce me to 50% starting october, but due to the notice period it had to start in December, and every month since they give me a new agreement extending things another month, so I am still at 100% until last month, now 50% + another 50% at a subsidiary company!
(I have too much to do, and one guy just left, so that is good for me)
c) Once you are at 50%, you can get unemployment benefits, but need to start looking for work as soon as you receive notice. So while I am in perpetual notice, I keep applying just in case things get to that point, 2x/week.
Tom