This answer will be in two parts, one to address your specific question ("What the heck do I do now?") and one to address the organizational failures that got us here (I don't want to let them go as they're all too common, and not enough people address them so they keep happening).
Your company has failed you, repeatedly. As a software developer (not a 'tech lead' or an 'architect' or 'manager'), you're not supposed to have managerial oversight (and more importantly, responsibility) over a project.
There were many failures here, none of them your responsibility, but for the good of the organization, need to be addressed:
- Your manager should not have 30 direct reports. The CEO shouldn't have 30 direct reports. Military organizations do a lot of research and study into the optimal reporting structures to both enable initiative at the lowest level while ensuring each manager has a focused grasp on what's going on and who is doing what and why. The Army in particular recommends 7-10 as a squad size (the lowest level discrete element), and even in that there are generally two fire teams of 3-5 people that work in concert.
No supervisor in the Army manages more than 7-10 people; that's from the Secretary of the Army on down.
They may be responsible for the actions of their unit; but they do not directly supervise all the people in their unit -- they'd never be able to get anything done if they did.
This is organizational failure #1: Your organization is not sized for success, and its organizational chart does not reflect a balanced organization. This is your CEO's fault.
- Your manager should never code. Ever. Even if you took the 30 direct reports down to a manageable 7 +/- 3, your manager would have their hands full in a functioning organization just keeping up with that and with planning and managing up. One of the telltale signs of an inexperienced manager is that they think they should still code. They shouldn't. Their job is now to ensure the success of their people and the success of the organization and projects under their purview. That's it. It's not to push production code, it's to create and maintain the conditions where their projects and people are successful.
That's the second organizational failure.
- Your manager's manager failed to effectively oversee their work. Someone besides you knew about this project, someone with managerial oversight and responsibility. That person failed, and the person who supervises that person failed. If the project crosses departments, then you can add to the failure all those managers in the other department that didn't do their part to keep up with what's going on.
That's the third organizational failure.
What you should do
There's an old saying:
"It is difficult to get a man to understand something when his salary depends upon his not understanding it."
Which means you're not going to be the one to bring all of this to light, no matter what you do. Your boss's continued salary depends on his failure to see this as an issue; as does his bosses's boss. You make a nice fall-guy (I wish fall person had the rhetorical flourish of fall-guy).
It's time for you to go email diving. Into every email you sent or received on this project. You need to produce all of those emails along with a timeline of what you knew and when you knew it.
You will then send this to your boss, your boss's boss, and your boss 3 removed.
Indicate that you regret that this fell through the cracks; and you'd love to know how to best fix it.
You should also prepare to be fired (if you work in the US).
You did nothing wrong here; but you end up on the bad side of a bad organization. This was not your fault, and you shouldn't feel responsible for this failure.
The reason why I say you should send all the evidence and data to your boss and their boss and their boss is because you're sending a subtle message that this failure transcends layers; and that you know it transcends layers. They don't like black-eyes, and so it might induce them to give you a severance to keep you quiet about it.