Magic the gathering: If a creature's toughness is below zero, what happens?

Question

If for example i were to play a Dauthi Horror which is a 2/1 with shadow, could i play the enchant creature card "Twisted Experiment" on it? Twisted experiment gives the enchanted creature "+3/-1." Hence making the creature a 5/0

Answer 1

(edited for relevant content)

420. State-Based Effects

420.2. State-based effects are always active and are not controlled by any player.

420.3. Whenever a player would get priority (see rule 408, “Timing of Spells and Abilities”), the game checks for any of the listed conditions for state-based effects. All applicable effects resolve as a single event, then the check is repeated. Once no more state-based effects have been generated, triggered abilities go on the stack, and then the appropriate player gets priority. This check is also made during the cleanup step (see rule 314); if any of the listed conditions apply, the active player receives priority.

420.5. The state-based effects are as follows:

420.5b. A creature with toughness 0 or less is put into its owner’s graveyard. Regeneration can’t replace this event.

As soon as it has 0 toughness it dies. You cannot respond to this once it has 0 toughness.

Answer 2

The second a creature has taken damage equal or above its toughness, it dies, right?  Therefore, if a creature has 0 toughness, it will take 0 damage to kill it.  Therefore, at any time it has taken no damage, it is dead.  Therefore, it is dead right then and there, right?

Answer 3

It seems to me that "twisted experiment" is a strange card since it reduces the toughness permanently, or at least for as long as its enchanting something. I'm not really sure what happens but I think if you block it will always die.