Nowhere. As of now there is no such a thing as a Mega Salamence.
There is no such thing as a mega stone in Pokémon Pearl. Mega stones were first introduced in Pokémon X and Y and are required for certain Pokémon to Mega Evolve, which is a temporary new evolution stage.
That's unknown. Arceus is already too strong as it is now, it doesn't need a Mega Evolution, and holding a Mega Stone would prevent the use of the Multitype plates anyway.
No, Mega Evolution can only be used during a battle.
Xerneas does not have a Mega Evolution.
Sceptile learns false swipe at level 59. This Pokemon evolves from Grovyle at level 36. Sceptile is a final form but using a mega evolution stone, does have a mega evolution.
No, because Mega Evolution wasn't introduced until Pokemon X and Y.
Have it hold the Sceptilite, then press "Mega Evolution" during a battle to Mega Evolve it.
The chances of pulling a Mega Evolution card from Pokemon packs with Megas vary depending on the specific set and rarity of the card. Generally, the odds are around 1 in 72 packs for a Mega Evolution card.
Seviper does not have a Mega Evolution.
Lapras cannot evolve any further in Pokemon, nor does it have a Mega Evolution.
Aerodactyl doesn't have a traditional evolution. It does, however, have a Mega Evolution, which you can evolve it to by having it hold the Aerodactylite and selecting "Mega Evolution" during a battle.
Currently, Pikachu does not have a Mega Evolution form or haven't announced a Mega Evolution. Relating to Mega Evolution, Raichu, however, will have a Alola Form, exclusive to the upcoming Pokemon Sun and Moon.