When returning as AVIS car at LaGuardia does the shuttle bus go directly to the terminal or is it still necessary to transfer from a shuttle to another bus at the old marine terminal in the back of the airport in order to get to the terminal?

When you return your vehicle at Avis, then you'll need to take the Avis shuttle bus back to the transfer point at Terminal A and take a red or blue route shuttle bus to the appropriate airline terminal.

How often do the red/ blue route buses run? Any schedule?

Yes, same,for 5 yrs

I believe it's the red route you're gonna have to take