1- Product Key sent via Email buyers is from address. rcowebmaster@ip-MyServerIPAddress.secureserver.net .LimeLM is using myserver IP , that i don't wants to be.
Right, you should look into your PHP configuration. It's (PHP is) using whatever email it can find, and apparently that's the one it could find.
2- FastSpring serves the same purpose of sending product key to buyer. If LimeLM is doing same thing at it's own, what advantages do i have for using fastSpring.
No, FastSpring is a separate payment platform. It's just an option.