1. Contract, contract, contract. Both with the client to avoid going directly under severe penalties, both for employees (even subcontracting entities or freelancers) to scare them off such behaviour. And of course, keep your guys happy and on interesting projects, that will help nowadays more then any penalties.
2. No. Either you're end to end responsible for the development and your contract says so (and allows you for subcontractors) or you're only providing on top of cream management service, in that case they should have separate contract with those guys.
3. Design, Scalling architecture, marketing mix, R&D....you can add a lot of flavour into this. Depends on your skillset.
4. A lot. One of my companies was focused as "Foundation house", project based only, including full business development & planning services, goverment discussions around regulatory issues and management of other third parties.
Call me if you want more info, that's quite a broad topic and you probably need to have it sorted to have good client value proposition.