Does my active Sylveon's "Safeguard" Ability prevent Cornerstone Mask Ogerpon's "Demolish" attack?
No. Safeguard is an effect on Sylveon, and Demolish ignores effects on your opponent's Active Pokemon.



×2Safeguard
Prevent all damage done to this Pokémon by attacks from your opponent's Pokémon ex and Pokémon V.

Ghost EyePut 7 damage counters on your opponent's Active Pokémon.
This Pokémon lives in dark places untouched by sunlight. When it appears before humans, it hides itself under a cloth that resembles a Pikachu.
Does my active Sylveon's "Safeguard" Ability prevent Cornerstone Mask Ogerpon's "Demolish" attack?
No. Safeguard is an effect on Sylveon, and Demolish ignores effects on your opponent's Active Pokemon.
If I use Kyogre EX's "Smash Turn" or Celebi EX's "Wind Whisk" or Gengar EX's "Dark Corridor" attack, can I switch it with my benched Sigilyph that has the "Safeguard" Ability?
Since the attacking Pokemon is the target of the switch effect, Sigilyph can be moved in this situation.
If I use Victini-EX's "Turbo Energize" attack but choose to attach the two Basic Energy cards from my deck to Sigilyph with "Safeguard", what happens?
Sigiliph is a valid target for the choice of Benched Pokemon, but since Safeguard prevents all effects of attacks done by Pokemon-EX the energies would be discarded rather than attached to Sigilyph.