Starcraft – Is it possible to go back to Heart of the Swarm campaign missions after continuing

The campaign lists 26 total missions, though some of these are "A or B" missions. Though you may only complete one of these missions for credits / research / campaign credit, the other, alternate mission is available on the console on the bridge to be replayed any time you like for achievements, etc.

Given the nature of those "pick one" missions, I believe there are 29 total possible missions +- a bonus mission that is only unlocked during special circumstances. (Or there may be 26 missions +- the bonus mission, I'm unsure how the "Campaign progress" bar, x/26, accounts for the branching scenarios.)

If you press "Continue Campaign" after finishing the last mission, it will bring up a window similar to the Missions Archives you could access during the game. For all of the missions where you could choose one mission or the other, both options will be available to play here.

The only mission that you can't play if you missed it is the Secret Mission. To do that, you'll either need to start a new campaign or load a save game from before you finished the mission where you unlock the Secret Mission.