Sending 5 cards to the Graveyard is a cost. The player chooses how many cards to send.
The ATK increase remains as long as this card remains face-up on the field.
Sending the cards to the Graveyard is a cost, so the actions are not simultaneous. The ATK is gained when the effect resolves.Furthermore, if among the cards that are sent as a cost there's a Peten the Dark Clown, its effect cannot be activated.
↑Konami FAQ: Laval Blaster's text reads "You can send up to 5 Cards from the top of your Deck to the Graveyard" and "this card gains 200 ATK for each "Laval" monster sent to the Graveyard by this effect". Are these actions simultaneous?
Furthermore, if among the sent cards there's a Peten the Dark Clown, can its effect be activated?