I spend a lot of money and time waiting for a claim to be fabricated, and all of a sudden the claim seems to be worthless. I own of a 2-county duchy because the emperor granted me it. I want the other county and want to press a claim. However, there is no war-option in the character profile of my vassal, nor that of her steward. I can't even select it in the emperor's profile. Is it because the HRE is still on the Low Crown setting? Is usurping the title the only way? Because I can't do that. It is forbidden by the 'rules'.
This other count is your vassal, right? And you now have a claim on it? In that case, click the county's flag. At the top of the screen, on one side, it should say "History" and "Claimants", or something like that. Somewhere on the other side it should say "Usurp", "Revoke", and some other stuff. Hover over the revoke button. It should say something like: she will have -80 opinion of you but because you have a claim your other vassals will not hate you; there is a 5% chance that she will revolt (note: it might be 100%, because she makes up half of your realm. But it usually seems to be 5%).
Click that button.