Because you lose compounding, not just contributions. Money invested early has more time to earn returns on its returns. The gap grows faster than linearly the longer your total time horizon is.
That depends entirely on what you're investing in and your own risk tolerance — this calculator doesn't assume any specific market or asset class. Many long-term diversified stock investors use a range of historical average returns as a reference point, but future returns are never guaranteed.
Not directly. If you enter a return rate that's already adjusted for inflation ("real return"), the result will be in today's purchasing power. If you enter a nominal return, the result will be in future (inflated) currency.