Correct me if I'm wrong, but isn't it possible to unbond a shardblade by destroying the gemstone in the blade, like what Adolin did in the first duel for shard? If this is correct, then the bonding issue isn't that great. Liss lends the blade Talak, and she doesn't know he dies so never knows not to summon the blade back until it's too late and the gemstone has been destroyed and the bond severed.
It's also possible Helaran never truly owned the blade himself, but was simply borrowing it because he thought he may need to threaten his father. Shallan has no way of knowing if it's actually his blade, or if he'd simply been borrowing it at the time. As stated, he'd received it from a group of people, but that may have simply been a loan. I can't imagine many people go round simply giving away shards. Helaran may have ended up in Liss' service and borrowed the blade from her.
We've also seen that outside the king there are a few people who loan out there shards. The highprinces who don't go riding into battle often loan them out to soldiers if they believe it can help the battle, knowing full well they can call back the blades at any time if they need to. Given the fact that the highprinces never let the people borrowing there shards too far from their sight, it would be reasonably safe to assume they'd need to see the person with the shard fall if they wanted to summon the blade back without potentially losing it, or taking it from a soldier who was using it.
Finally, combat rules (for all they are actually followed) dictate that whoever slays a shardbearer takes their shards. Depending on how you interpret shardbearer, it could either mean the person who the shard is bound to, or the person using the shard at time of death. If you're strictly following the rules then a person wearing/using the shard, but not bound to it can still lose the shard if they die while wielding/wearing said shard.