Start with what the money is for, such as a deposit, a repair fund, or a planned purchase. Use a price you can check, and add some room for costs that tend to appear late in a plan.

The monthly deposit is simply the amount still needed, divided by the number of months. The calculator works that way when the rate is zero.

Within the same timeline, yes. Interest does part of the work, so you have to deposit less yourself.

No. Savings rates can change, and investments can lose value. Use a rate you can reasonably expect, not an unusually high advertised rate.

A longer timeline spreads the goal over more months, so each deposit is smaller. A shorter timeline needs larger deposits to reach the same amount.