If the theory is descent with modification and the fact of such descent is observed, indirect evidence is observed, the the theory is validated, yet again. This validation is proof in your vernacular.
It is not. Try to understand the difference between induction, which science operates with, and deduction, which abstract formal systems operate with. A deductive demonstration from formally accepted axioms and postulates is a proof, and is more or less incontroveratable within its formal framework.
An argument from induction isn't in the least incontrovertable. It is a statement that we have faith that what happened before, will happen again. Which we believe until the day it doesn't work, and then we find some other, more comprehensive statement. This is how science works. Attacks on biological science that don't account for this, are like peeing to windward to stop a ship from moving.