Trading plan is basically your strategy. In every plan, back plans for unexpected incidents should also be considered such as contingency plan. Taking note of the market distinct characteristics such as unpredictability and volatility. No fixed strategy for daily trading, it is always "it depends on the situation".
Right. Be flexible. As long as you know what you're doing then any plan would be okay. Having a plan beforehand lessens your tendency of being emotionally drawn, which is one of the reasons why traders get frustrated.
Stick with the trading plan which should really be always part as a trader or investor even simple decision makings in life does really need some plan.
Not having any plans or methods on how to achieve your goal would really able to mess up along the way. Troubles and problems cant be avoided and as you said
we should be flexible on all possible sorts of problems.