There will be spoilers here so be warned:
spoilers:
I think the Death Note with L and Light is fake. This means L knew the whole time he wouldn’t be killing Light.
I think in the preview it most likely will be Near dressed as Mello. I go back and forth on this one.
Pretty sure L will die in the few minutes before the others arrive. So either Mikami will write his name, or Light will on the paper in his watch once he realizes.
I also think L meant to tell no one else and keep Light near him on some pretext hoping to get him away from the Death Note. However, it seems obvious that he knew this might not be possible hence why he warned Near that it might be Near to stop Light.
I don’t think Near/Mello killed anyone. The man seemed to die of a heart attack which likely means Mikami was dealing with loose ends.