On the day before the reward decreases from 50 to 25, there will be approx 7200 new coins generated out of a total of 10.5 million. Less than 0.08% extra. On the following day there will be approx 3600 new coins generated out of a total of 10.5 million. Less than 0.04% extra.
The change from 0.08% to 0.04% per day won't upset the markets, and the miners will have known about the coming change for ages, and will have factored it into their plans and budgets.
This is not something to sweat about.
Halving the reward is the equivalent of having half the gold content in your ore disappearing over night. This will cause most miners to become unprofitable, so block generation could crawl to almost a halt, making the next 2016 blocks very long and painful.
Doubtful. At worst, half the miners will give up because they suddenly became unprofitable, and the interval between blocks would stretch to 20 minutes.