Don't fall in love with me yet, we only recently met
1. In order for Ren to flip a heart card by using a card of the same color in the poem, does a single card from the poem have to match the heart card color, or every card from the poem? If only a single poem card has to match, is the heart card flipped as soon as a color-matched poem card is played, or only once the poem is complete?
As soon as a card is played into a row where it matches the heart card, the heart card is flipped.
2. If Ren has already flipped a heart card by color-matching a poem, she turns the card to indicate that it wasn't revealed through Feth reliving a memory. If Feth later relives a memory for the same color, the rules state that tapped cards in the poem row are untapped. Does that include the heart card, so that it will now be added to Ren's score pile at the end of the dream?
No. Once a card has been revealed by colour-matching, it cannot be scored.