If a court determined that he breached, there could be damages for losses to the company based on his breach, i.e. it could go beyond him just giving back what they gave him. This is really the kind of thing where you need a lawyer's guidance based on very specific facts.
I think you might be over-reacting to these provisions. It's entirely normal that you sign over ownership of the code to the company in exchange for your equity. That's in every form stock agreement. Are you using an experienced startup lawyer? It sounds like maybe you are not. You don't need a big fancy law firm, but you do need a lawyer who is very familiar with startups and standard terms. You should confirm that your lawyer has worked with many clients who have negotiated with startups.
Not so clear cut. You would need to be very careful if you tried to pull this. Look at the facts he explained. The investors are just coming in now, and the founders are saying that the stock agreement depends on the investors' lawyers. That's actually pretty reasonable in a situation where you have a bunch of inexperienced founders getting money from experienced investors. The investors might force the founders to agree to different terms in their own stock agreements as well. It's not at all clear that the founders have breached anything yet. If he was to take the code elsewhere, that act could actually be considered the breach.
You should really talk to a lawyer about this. Oral agreements can be binding and enforceable. If this project is big enough that the founders would actually fight you, then you definitely should not assume that no written agreement equals no deal. Unfortunately, the short answer is that it's complicated.