If I don’t get impetigo treated, will it heal on its own?

May heal, not usual. It depends on the size, if it is very small and early stage, the way you clean and wash it and how you avoid spreading can affect the healing. It is very contagious and if you do not control that, it will spread fast and you need the treatment. Most of the times you need treatment but sometimes the nature of the body and immune system takes care of it.
Usually not. Often lingers, spreads, and may become more serious, causing cellulitis or abcess. If caused by strep, it can cause glomerulonephritis, or, if spreading to the respiratory area, rheumatic fever. Often spreads to other members of the household, or came from other household member carrying the bacteria assymptomatically.