They have little to no chance if you don't raise the temp, don't worry about raising it slowly, just try not to overshoot.
You can't correct the past, any survivors will undoubtedly be late. Be very careful to monitor the air cells and adjust your humidity based on what you see today based on the current development stage of the eggs. At day 12, the eggs should be 3/4 full of chick with 9 days to go. I would guess that your survivors are probably at day 7 or 8 stage, and the air cell should be sized for that stage, not how many days they have been in the incubator. You may need to raise the humidity to ensure the air cells are not too large, too early.
If your eggs are at day 7 stage, expect an additional 12-14 days of incubation and time lockdown accordingly.