First off, Skill Drain negates effect cards. That means as long as Zombrya is on the field and face up, he has no effect. Even though he states on the card that he cannot attack directly, that is a condition that is negated since it is part of his card text as an effect monster.
Second, once Zombrya is destroyed or removed from the field, he would be unaffected by Skill Drain, and resume back to being unable to attack directly, but he is no longer on the field.
Third, Skill Drain would also negate Dark Ruler Ha Des' effect and condition for being Special Summoned from the Graveyard, but its not in the Graveyard, so it really doesnt matter. When Dark Ruler is removed and sent to the Graveyard, it is once again unaffected by Skill Drain, and cannot be reborn because its not an effect (its a condition) that can be negated while in the Graveyard; only on the field and only because its not a maintenance cost or cost effect.
Fourth, Monsters successfully summoned by Magical Scientist are under the condition of the intial summon, and Skill Drain cannot negate the condition that brought them to the field. It can only negate the effect of the actual card that was summoned.
So, in conclusion, you cannot "really" negate a condition using Skill Drain. It is the actual effect monster that loses his card text (unless its a Maintenance Cost) and along with it, the condition or restriction, as long as it is face-up on the field.