Legal Question in Civil Litigation in California

First thing you should do is make sure you have named every entity properly or you may have problems down the line. No, you cannot serve instate persons by way of registered mail. You have to determine what type of entity it is, partnership, corporation, etc., and that determines service. A corporation is served, among other methods, by serving their registered agent. A general partnership may be served by serving one of the partners.

No. You must personally serve an individual or substitute serve if you have diligently tried to serve and cannot find him/her at home or work. You must serve the registered agent of a corporation or LLC. You can find out who that is at Secretary of State.

Yes, if the defendant is outside California. If the defendant is in California, you can mail him the summons, complaint, etc. along with a blank Notice and Acknowledgment of Receipt. If he fills out the notice and returns it to you, then he will be deemed served. But he's under no obligation to sign it. If he doesn't, you will have to serve him by other means -- but you may be able to make him pay the cost of doing so.

Note that proper use of a Notice and Acknowledgment of Receipt involves other steps. I have not tried to detail all of them here. You should make sure to take all the necessary steps before you try to use that procedure. If you do it incorrectly, then there will be no consequences if the defendant doesn't sign the notice.

I agree with Mr. Hoffman.