I could see a point in the proposed change, but I’d drop the invulnerability completely.
Classically, pirates can warp their (in)famous bomber, dessie, battlecruiser or whatever they like swarm onto a sieged highsec crab, tackle and engage it, and contact the crab offering it to ransom itself out. The pirates can prepare this part, i.e. open the private chat page or prepare an ingame mail, where they state their terms, before starting the engagement. However, the crab will get a potential ingame mail receival notification only among a dozen “kill permit received” notifications, potentially not noticing the the mail, while private conversation requests tend to appear in the background of other windows (this needs to be fixed nevertheless). Nevertheless, the victim has to notice the pirate fleet’s attempt to contact him, read their terms and “process” their demands in very little time, before the pirates can stop their aggression. In highsec, CONCORD is more or less forcing the pirates to make sure there is little time to accept it.
Even if the victim was to accept the demands of the pirates, the situation where some members of the pirate fleet have their client crash or lose internet connection, in which case their ships continue the last action they were doing while their client was healthy, and continue shooting the victim despite it having payed the ransom, has to be mitigated. Right now, the pirate fleet commander can pay back the ransom or even the ransom as well as compensation for the ship, but as @Syzygium has pointed out, the incident still dishonors him and his corporation.
I’d rather see a built in interface to streamline the ransoming process. Accepting a ransom offer should spin down all aggressive modules on all aggressed capsuleers, and, to all fleet members, disallow spinning them up (again) for 10 seconds. Automatically Spinning down the modules addresses the disconnected client issue. The 10 second timer is in order to mitigate some BLOPS fleet or dessie gang that has it’s usually present multiboxers alt tabbing through their alts in the moment the fleet commander accepts the ransom spin down his mods on the first x alts but just aggress on the x+1 th alt in the interval beween the FC accepts the ransom and the multiboxer realizes it was accepted. If the pirates feel like dishonoring their FC’s ransom deal, they can drop fleet and re-aggress, or bring some out-of-fleet members in the first place, that will not be distinguishable to the victim, so it pays a bribe and gets wrecked nevertheless.
In case the gankers don’t ask for ransom as they start aggressing, a victim might want to offer some instead. However, the local chat has been broken for longer then it has worked in the last 4 years, so it can’t be reliably used for communication (this needs to be fixed nevertheless), while starting a private chat might not be straightforwardly possible in the event the victim doesn’t know who of the pirates to direct the offer to. In that case, using the streamlined ransom interface should automatically redirect the offer to the FC.