Tributing "Lava Dragon" is a cost to activate its effect. Its effect does not target, so the Dragons are chosen at resolution of the effect.
If you do not have at least 1 appropriate Dragon in your GY and 1 in your hand, you cannot activate the effect of "Lava Dragon".
If the effect of "Lava Dragon" resolves and you can no longer Special Summon an appropriate Dragon from your GY and 1 from your hand, you cannot Special Summon any Dragons for the effect.
If the effect of "Lava Dragon" is activated while "Necrovalley" is activated, the effect does not resolve.