My teacher always says that a strike always has the maximum power it can have given the exact circumstances, and that my opponents body will always break at the weakest point for grappling. is there any evidence in martial arts or elsewhere to support this saying?
The first statement is technically true. Due to the essentially deterministic nature of the universe, for a given environment, given execution of a strike, and given disposition of the opponent, the strike has precisely one execution, the one happening, with one result, which is both the maximum and minimum power it could have.
The second is kind of true as well. For that individual situation, by definition, the break happens at the weakest point, which might be psychological. It's basic structural dynamics that a single point fails, and generally cascades into complete failure. That said, it implies that that point is generally weak, which I feel is less true. As with the first quote, it's situational. That first failure will depend upon the execution of the technique, the reaction of the defender, and aspects of the fight, possibly including earlier damage.
The more salient question, is what meaning your teacher intends to convey with these statements. One possibility is that they are just trying to sound profound without a deeper meaning. This is sometimes referred to as an ice cream koan, a play on words on "Zen koans". Another possibility, is that your teacher is trying to use this to make you think more about the technique, thinking less of it as a matter of "this technique is more powerful than this other one", and more that the situation where they are used determines how useful they are. Similarly, thinking of a grapple succeeding at the then weakest point of your enemy, makes you think about the entire chain of dependencies involved, and that all it takes is the failure of one. Ultimately, this may be a question better suited to discussing with your teacher, as to what he truly means by them.