Did the ancient Greeks consider their deities to be omniscient? Hence does Greek mythology claim to be the definite truth like other religions often do and might this leave more room to question and even apostatize?
Not even close.
This can be shown by, for example:
Hephaestus only discovered Aphrodite and Ares affair after Helius saw it and told him. Then Ares and Aphrodite had no foreknowledge of the net trap: https://aquileana.wordpress.com/2014/10/02/greek-mythology-aphrodite-hephaestus-ares-and-her-other-lovers/ Similarly, Hera is fooled by zeus pretending to be a cuckoo, and doesn't find out about his many children (at least for a while)
Prometheus tricked Zeus by hiding meat inside an animal stomach, and bull bones in glistening fat. Zeus chose the wrong one. (Allowing for the fact Hesiod implied Zeus knew, "in mainstream versions of the story Zeus was actually deceived, and that Hesiod is trying to be pious by changing the story to make Zeus look better" http://blog.world-mysteries.com/science/the-prometheus-myth/ https://en.wikipedia.org/wiki/Trick_at_Mecone
Thetis didn't realise Achilles vulnerability after dipping him in the Styx https://www.greekmythology.com/Myths/The_Myths/Trojan_War/trojan_war.html
Kallisto manages to hide her affair with Zeus from both Hera and Artemis, at least until she became too pregnant for it to be obvious: http://www.theoi.com/Olympios/ZeusLoves3.html#Kallisto
Poseidon doesn't spot odysseus after leaving Calypso's island for 18 days, when he spots him on his way home: http://www.sparknotes.com/lit/odyssey/section3.rhtml
You can find a lot of other examples, of course but the gods don't appear to have anything approaching omniscience.
The Fates are omniscient. Clotho spun the tread of fate. Lachesis measured it. Atropos cut it. All the while observing the would and knowing every thing between the three of them. Except what would happen to themselves, for Apollo, as a mortal, got them drunk and forced them to tell him how his human master could avoid his fate.
The Graeae could be considered all knowing as well. They knew the three items needed to kill Medusa and where they were. But, they could not see the Perseus would take their eye and tooth.
I realize the Fate nor the Graeae are gods.