Why you don't save before and during the missions is a question. Basic tactics. I had 4 different saves for the quest before I settled on which route I wanted to take, one before I even went on it.
Well, I do, and it works... for a while. But the amount of saves you can keep in this game is limited (even by keeping savepoint files in different archive folders as I do), so in the long term it soon becomes unfeasible to keep such a fine grain for every significant quest, mission, or plot/character development. As a rule, every savepoint of mine eventually ends up being a milestone for at least an handful significant events, so say savepoint 10 (effectively 22) in the second TiTS saves archive may concern Syriquest, the Pollen Festival, getting another dozen kids from Frostwyrm and LK Lah, and Fen knows what else. IIRC I have accumulated about 24 savepoints total from beginning of the game. And I usually have to keep at least 2-3 savepoints free to test different playthrough variants during quests, manage save scumming and save editing, keep alternate Fem and Herm versions of the character to tap different scenes, and so on.
To sum it up, what you describe is feasible if the quest concerns the last update and has been played recently. Not so much when the quest is already a few updates and weeks/months old, and suddenly new information that makes the previous playthrough suboptimal becomes available to me. At that point, save editing, if I know what to do, may often be preferable to set things right for a single plot hook rather than backtracking and losing who knows what else. Going back to not-so-recent saves is the last-ditch solution when I somehow screwed up so bad the playthrough cannot be recovered in any other way.