If someone had excessive vasoconstriction in their arm but not anywhere else would the blood pressure reareading be wrong and show too high?

Giant cell arteritis: Several arterial diseases and congenital anomaly can do that. Rechecking is important.

Yes and No: They would be wrong and be too low. If your brachial artery, the one that BP cuffs measure, is constricted, the BP is artifactually low. The real BP is a core bp-the BP inside your left ventricle and aorta. That's the same as your brachial artery if there is no constriction.

Low: If there is significant vasoconstriction/spasm, blood pressure readings would be lower in the affected arm.