If I'm playing Ivern legend and I conquer the Forgotten Monument battlefield before the third turn, if I turn it into a brush do I still score for that conquer?

No, you do not score for that conquer.

According to Forgotten Monument's ability: "Players can't score here until their third turn." Because "Conquering" is a method of scoring (Rule 464.1), you are prohibited from performing a Conquer action on that battlefield until the condition on the card is met.

As stated in RiftJudge FAQ #9332:

"You cannot conquer Forgotten Monument before your third turn, so it cannot count as a conquer for any triggers... If you attempt to move a unit to the battlefield before your third turn, you will not be able to score or conquer it, and therefore no 'When you conquer' abilities will trigger."

Because the action of conquering (and the subsequent scoring) is prohibited by the battlefield's text, your ability to trigger Green Father also fails, as it requires you to "conquer or hold" the battlefield. You cannot successfully complete a conquer to trigger that ability.