Yes he can. It's why Ford pardoned Nixon.
Exactly. What happens when a sitting president is impeached and convicted is that he is removed from office.
Once removed from office, if the new president wishes the old president can then be prosecuted for any federal crimes that are included in the “high crimes and misdemeanors” he was removed from office for.
Obama is already out of office, so he would be prosecuted as an ordinary defendant in a federal court.