Personally, I think it's all valid, however, I don't believe execution will be as easy.
If players are not aware that a noob is being attacked, they won't reinforce the battle, the noob dies.
If players are aware and rush to the noobs aid, it can take them a good 5-10 minutes to the battle and if they are running, another 5-10 for them to get shots on the enemy. In that 10-20 minutes, the noob will be dead.
"Reinforcement Missions" IE noob got tagged inside the reinforcement zones, need to have a way for players to know where they are, sailing around for 10 minutes trying to find a white X against a blueish white sky, its hello kittying annoying and means the noob dies.
However, I believe there are a few options.
AI reinforcements primarily focus on de-masting/de-sailing enemy ships. Most AI does not follow the game meta and is then easy to deal with. If they are constantly picking away at your sails, you'll have a hard time dealing with that player trying to make a run for it.
AI supports the defending fleet at a higher BR, as you said 1.25-1.5, till players are able to join and support the noob.
Reinforcement Missions could have a big red X for them. Players would know it's a player in need of help.