WOAH, so you CAN save scum in this game, sort've.
So a few posts above I posted that I ragequit after losing my commander in a battle. When I turned the game back on, I was able to resume the mission from the last turn when I got back to where I left off before I died.
When you choose to Exit from the Mission menu. This basically creates a save state of wherever you are.
You can then just go back into Campaign and resume from where you 'Exit'ed from. So you can play ahead and if things go south, just immediately Home Menu and Quit. When you resume the mission, you'll be back in the exact spot from where you 'Exit' from.
So effectively, as long as you 'Exit' from the Mission menu every few turns, you'll create a save state, and then to return to that save state just Home Menu > Quit (this is on Switch).