On the string length, the answer is yes and no.

These bows generally perform best when shot at a certain brace height. So, while it is true that changing the string length will affect holding weight at a given graw length, it is not an acceptable way to adjust draw weight.

The same model can come in different weights, but that is done by varying the stiffness of the limbs. I have never heard of an adjustable-weight recurve, but maybe they are out there. I know there are takedowns that you can just substitute heavier or lighter limbs.