1. Yes, if the post is at risk of redundancy you can be notified you are at risk even if you're off sick. Otherwise people could just avoid redundancy by going off sick and leave the conscientious ones who turn up for work to be put at risk.
2. Yes you are entitled to be paid for the notice period stated in your contract provided it's compulsory redundancy, but not if you take voluntary redundancy. I can't comment on the entitlement to redundancy pay without knowing the agreement in place but any predundancy would be in addition to notice pay
3.Yes, I understand you can get a partial severance payment from your employer in these circumstances to reflect the reduction in pay but it will be taxable- the tax free allowance of �30,000 won't apply in this case as you are still working. I doubt statutory redundancy pay would be paid too if that's applicable. However i'd check with your union/HR dept/manager