God questions.
Even the forbidden tree was good in and of itself in that garden.
What was forbidden, what was bad, was the ACT of any man eating from it. The ACT of doing so was forbidden. That did not in any way take from it the goodness in its creation. It was functioning perfectly as designed, as were all the other plants in the garden. It's inclusion in the garden fit in perfectly with God's plan.
As to Satan, when we read the narrative, it seems obvious that Satan had observational skills about what was forbidden to the humans, and so he monopolized on that to implement his plan, which so happened to fit in perfectly with God's own plans. Satan, in spite of his inherent evil, is still a pawn in the hands of the Most High, for he cannot do anything that would disrupt the Lord's plans for mankind and the earth.
As to when the Lamb was slain...when it comes to eternity, when is then, and when is now? When we try to exercise that muscle between our ears about a realm we've never experienced, and therefore cannot know with any measure of reasonable assurance, then shrugging the shoulders is the best expression of what we think we know.
MM