I recently had a conversation with some players regarding the fine details of mana payments.

The relevent example: If a player proposes casting mox opal for {0}. Would there be an opportunity to pay mana costs (assuming no cost increase effects)?

under 601.2g
If the total cost includes a mana payment, the player then has a chance to activate mana abilities (see rule 605, "Mana Abilities"). Mana abilities must be activated before costs are paid.

but also

118.3a Paying mana is done by removing the indicated mana from a player’s mana pool. (Players can always pay 0 mana.) If excess mana remains in that player’s mana pool after making that payment, the player announces what mana is still there.

On a follow up line of reasoning, if you control Ashiok Wicked Manipulator and Yasharn Implacable Earth, can you cast fire covenant for X=0?
with the following rules

118.3b Paying life is done by subtracting the indicated amount of life from a player’s life total. (Players can always pay 0 life.)

119.4b Players can always pay 0 life, no matter what their (or their team’s) life total is, and even if an effect says players can’t pay life.

I would appreciate some clarity here, thanks