1. Pack as much relevant data per send as you can. Start to throttle back some only if you are encountering bandwidth problems.
2. You have it wrong. The renderer should render what it can as soon as it can. The only reason to force your renderer to wait is if you are purposely cutting back the framerate.
Interpolation should occur in the client side code per update. Use whatever came in from the network and do a "confirmed" position update on those entities. For the remaining entities, you will use position, time, velocities, etc. to give it an "estimated" position.