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A Schengen visa has both validity and duration of stay. You can stay as many days as the duration of your visa, within the time window of the validity.
Question 1: Yes. You can shift the dates within your validity perior.
Question 2: Depends on how much duration you have used up.
Question 3: Yes, as long as Poland remains your main destination. There could be problems if somebody thinks that you wanted to go to Germany all along, and left that out of your visa application. But minor changes to the itinerary are OK.