Is it possible that alagilles syndrome be mistook portal pulmonary hypertension?

Yes: I have not seen one but literature says that it can indeed cause portal hypertension due to the liver problem and blood return is impeded. The smaller pulmonary arteries can be deemed pulmonary hypertension unless left and right heart cath can be done to prove the pulmonary circulation is ok and only the larger artery involved, cath may be difficult due to the pressure and size of vessels.