Light and darkness dragon's effect

Since LADD's effect is mandatory and he only has 2800 atk what happens if he negates spells, traps, effects until he has 300 atk what happens because he can no longer pay the cost to negate but it is mandatory. Secondly he can't be special summoned but if he is tributed and then destroyed can he be reborned with monstter reborn or Darkness metal dragons effect after he was summoned or not?

He can't be Reborned, revived with REDMD, etc. "Cannot be Special Summoned" means just that, man.

As for the first question, when his stats are too low, his effect will activate, but since he can't have the ATK/DEF decrease during resolution, he won't negate. And actually, his final negate brings him down to 800 ATK, since at that point he has 400 DEF, 100 below the required decrease.

Okay thanks for the clarification