Learn Croatian: Tvoja tvrdnja nije točna. - Your claim is not correct.

Yes, they are often interchangeable, but there is a slight nuance. Točna usually means accurate or factually correct, like a correct answer on a test. Ispravna leans more towards proper, functioning, or morally right, though it is frequently used for statements too. Here, točna emphasizes that the facts of the claim are wrong.

What is the difference between a tvrdnja and an izjava?

A tvrdnja is specifically a claim or assertion, meaning it is something you are arguing to be true. An izjava is a more general statement or declaration. Since the sentence is actively judging whether the point is factually correct, tvrdnja is the most natural fit.

Since nije is a form of the verb 'to be', does it have to follow the strict second-position rule for clitics?

No, it does not. Unlike the positive, unstressed forms of the verb biti (like je or su), the negative forms (nisam, nisi, nije, etc.) are fully stressed words. They are not clitics, which means they do not have to sit in the second position and can freely start a sentence if needed.

How would the meaning change if I changed the word order to Nije točna tvoja tvrdnja?