Couldn't Misa have told Kira L's life span? In the manga Death Note, when Misa meets L for the first time, she has the shinigami eyes, so she must be seeing L's lifespan at the moment. L dies a few volumes later, so couldn't Misa have alerted Kira that L was about to die, and it wasn't worth much trouble?

Is this a flaw in the manga?

It might also be the case that since L was killed by a shinigami itself, the lifespan Misa saw with her shinigami eyes didn't account for that death.

Which of these possibilities is correct?
L was killed by a Death Note (Rem's). When someone is killed by a Shinigami's Death Note, the remaining life span is added to the Shinigami's life. This means the life span seen using the Shinigami eyes doesn't account for deaths caused by the Death Note (since that difference is added to their life). The important part of what the Shinigami eyes does for Misa is that she can see L's real name.