Flareon

When I try to attack, does Marowak's "Bodyguard" Ability prevent me from having to flip a coin if my active Pokemon was attacked last turn with Silicobra's "Sand Attack"?

The effect of Sand Attack is on your Pokemon that was attacked, not on you. So, Bodyguard does not prevent you from having to flip a coin to see if the attack goes through

If your opponent used Sandshrew's "Sand Attack" on the previous turn, and you attack with Passimian's "Intentional Grounding" which requires you to discard a Tool from your hand or the attack does nothing, do you discard the Tool before you flip for the Sand Attack effect or after?

Flip for the Sand Attack effect first; if it's tails your attack ends immediately, or if it's heads you proceed with your attack as normal.