I'll answer these in order but keep in mind that, though I have contract experience having run a digital agency of my own where I subcontracted work to developers, I'm **not* a lawyer or accountant in any country so you'll want to check with either one of those before taking advice that could affect your business outcomes.
1) It depends on the laws where you are and if the client wants you to manage the project and any subcontractors like developers or if they're open to directly connecting to the developers. It also depends how the developers you choose prefer to work. But, it makes sense for developers to get instructions directly from clients. Just make sure whatever you do you build the process into both your client contracts and any project plan documents where that information is necessary.
2) This also depends on the laws where you are and on what your contract with your client (and subcontractors) says. If the SLA requires you identify any subcontractors, of course you should. But, if you're taking all responsibility for the quality of the developer's work (as well as the liability), then it's not necessary.
3) Once more, that depends on the laws in your country. You don't want to get into trouble with taxing authorities. But, this would mean referring clients directly to developers and letting the two parties create their own contract, meaning the client pays the contractor directly. Know what the implications for your enterprise will be, though. Will you lose money if you refer developers directly to clients? Will you get affiliate or referral fees for developers you refer to clients instead of subcontracting? How will this arrangement benefit all parties and will there be any financial (including tax) or legal drawbacks?
It sounds to me like you're looking for ways to separate yourself from your developer partners. That might be a good strategy for a variety of reasons (not the least of which is reduced stress of having to manage them).
Just carefully consider what effect it might have on your business and get everything in writing. Make sure your contracts reflect how you intend to work with your clients so you won't have more responsibilities than you planned to have. Review every detail of those and project plan documents for gaps.
Please feel free to set up a call for follow up questions.