Terraforming Mars

You must meet a card’s listed requirements before you play it; you pay its cost immediately when you play it and then resolve any immediate effects. (p.9)

Yes — spending 8 plant resources places a greenery tile, immediately increases the oxygen level (and your TR where applicable), and you remove the spent plant resources from your player board. (p.11; tile placement p.5)

In the Production step you first gain resources according to your production values, and then all energy is converted to heat by moving energy to the heat box (energy → heat). (production p.8; energy→heat p.6)

Each city scores points equal to the number of greenery tiles adjacent to it (including opponents’ green tiles); cities do not score for adjacent cities. (final scoring p.12; tiles p.5)

As an action on your turn you may claim a milestone by paying 8 MC if you meet its condition (first to claim takes it), and you may fund an award by paying its cost to put it up for end‑game scoring; only three milestones can be claimed during the game and funded awards are scored at the end. (p.11)