Shouldn’t Glendyn Ball have been protected from seizure and torture by guest right? If not, what’s to stop any host from claiming some pretence, like theft, as an excuse to harm their guests?

Guest-right is reciprocal, so it’s not like you could kill someone under someone’s roof and then scream breach of custom if the host puts you under arrest. At the same time, torture is a more serious violation, because it involves physical harm to someone who is supposed to be sacrosanct. 

So I would say that it’s probably acceptable in Westerosi custom that if the guest is suspected of breaching guest-right, they can be arrested but must be given a trial. As Catelyn did with Tyrion. And prisoners should not be harmed, as Catelyn insisted.