Nice, interesting interventions, i like your Murum Aries Attigit Chipla, it makes sens in a CoE setting even if finding a way to bring it in the system is not easy.
The main problem with Murum Aries Attigit is the way a settlement is controled in CoE. The order of things we know so far imply that a settlement is under the control of the county it is in and for towns and cities under the control of the mayor/magistrate which is an elected position. There are a few exceptions to that, if the town is part of a coup (the mayor signed the casus belli) the authority of the county does not apply anymore till it is re-enforced and the mayor striped of his title for treason, in the case of a power vacuum the count will appoint a new mayor that is either an other mayor in the county or a land owner of the town.
So, in the case of a legal conflict, Murum Aries Attigit could be translated by a surrender contract, that contract offered by the attacker to the town would in exchange from the surrender of the mayor title spare the town and create a power vacuum if the attacker is on the lord's side or force the mayor to sign in with the usurper in the other case. We could also add that if the mayor refuse to sign the surrender contract, it triggers an eviction vote where the town council has a chance to evict the mayor and appoint an other one who will accept the contract. A timer would go with such a contract to leave only a few days to the town to accept or definitely reject the offer.
About the various points raised about the ransom, i agree, that is why it would have to be limited to staged battle ( limit in time and space) as i was envisioning it, if an incapacitated character is recognized as a valid ransom target ( to be defined, but if it was limited to nobles and wealthy it was because they were the only ones viewed as able to pay a ransom) a bounty of war token is issued that will let you capture the target the same way a bounty-hunter does with a criminal. The captured target must then be brought diligently to the camp of the army, where the ransom contract will be created. there the captured character can either try to escape or be rescued if his side manage to attack the enemy camp. At the end of the battle if the captured character is still prisoner than he is presented the contract, if it accept it he is freed and must fulfill his part of the contract (pay the ransom) if he break his word it is the duty of the other party to enforce the contract, breaking a contract has an impact on your reputation as it makes you less trustworthy. If the character refuse the contract he than is killed or suffer prison as if he was a criminal according to the terms of the contract.
Chipla's Murum Aries Attigit roman custom brought to my mind an other custom of them, hostages.
After a conflict or negotiations romans were often taking hostages, children of nobles, that they brought back with them to be sure the other party would not break the treaty.
Hostages were no normal prisoners, they were well treated, not really restrained and often treated as a roman child ( a famous hostage was Attila for example). So a peace treaty or a surrender one could include hostages, children sent to the other party to be raised.
That would imply for the hostage to be given an addendum to his child contract, giving him a new room, and transferring some parenthood rights over the captors, like language, culture and profession teaching.... it does not change who control the child code and in case it is needed the child can be used as heir, how the player manage his life in the enemy's land is ground for great stories. When the terms of the treaty end the child is sent back to his parents and the initial child contract resumes. As children can not be killed or harmed we lack the option to do that if the treaty is broken but is that really important ?