Grolnok's last ability allows you to play lands and cast spells from among all cards you own in exile with croak counters on them, regardless of whether they were put there by the Grolnok that's currently on the battlefield or by an earlier incarnation.

Grolnok, the Omnivore's second ability exiles all permanent cards that are put into your graveyard from your library, not just those that you mill when a Frog you control attacks.