In the first example, you should just continue playing tit-for-tat, if the other player truly "slipped" and knows you are playing tit-for-tat, he won't defect after you defect, he will continue cooperating and make you cooperate.
The whole notion of pre-commiting to a strategy is to remove the human elements like "negociation" and "excuses". You are a machine and you play accordingly. If the other player is rational, he will cooperate even after he slipped and you defected.
In the first example, you should just continue playing tit-for-tat, if the other player truly "slipped" and knows you are playing tit-for-tat, he won't defect after you defect, he will continue cooperating and make you cooperate.
The whole notion of pre-commiting to a strategy is to remove the human elements like "negociation" and "excuses". You are a machine and you play accordingly. If the other player is rational, he will cooperate even after he slipped and you defected.