My understanding be that a player can use ONLY ONE octagonal Spade action per turn. Am I correct?
Yes! (rare exception are chaosmagicians with their strongholdaction)
Do rounds that yield 2 points per Spade give such the bonus ONLY when terraforming? (Since Dragonlords and Acolytes burn rather than churn, do they instead receive only their power tokens/cult points when they gain Spades by octagonal actions and 4-point earth cult bonuses?)
You gain the bonuspoints whenever you use a spade. For most races it is when they terraform with spades (but not with other effects like nomads sandstorm). When Dragonlords/Acolytes terraform they don't use spades so they don't get points. But if they use spades to get powertokens/cultsteps they get the bonuspoints because they are using spades.