Note that “Horus the Black Flame Dragon LV8” has the word “except” in its Summoning conditions, which means that even if Special Summoned properly, you cannot revive it with “Monster Reborn” afterwards if it is destroyed.
The “you” in “on your side of the field” is the current controller’s field, so as long as “Horus the Black Flame Dragon LV 8” is face-up, it is always “face-up on your side of the field”.
The ability of “Horus the Black Flame Dragon LV8” to negate Spell Cards is a multi-trigger effect that must be chained to the activation of the Spell Card. You can activate this effect multiple times in the same chain to negate the activation and effect of several Spell Cards that are activated in the same chain.
You can chain the effect of “Horus the Black Flame Dragon LV8” to the activation of a Continuous Spell Card, Field Spell Card, or Equip Spell Card and negate it. However, if the Continuous, Field, or Equip Spell Card is already active, you cannot later decide to try and negate its effect with “Horus the Black Flame Dragon LV8”. You can only negate it when it is originally activated.
You can activate the effect of “Horus the Black Flame Dragon LV8” during the Damage Step.