Regarding Lucifer and Adam, I find the answers speculative at best.
@Soldier's Psalm: Yes, Adam was to tend and protect the garden - but the Bible does not say he was to guard it from Lucifer, that is just one way this can be interpreted based on the presupposition that Lucifer had already fallen and was already roaming the earth.
@Jim an Apostle: You have speculated about free-will etc, and once again this is an explanation that might work - but I don't see any actual scriptural reason that this explanation is correct.
The entire Gap Theory rests on narrow interpretations of passages in poetic and prophetic books, and is just one way these could be interpreted. For instance, why assume Lucifer was on earth and was judged here at his fall?
There is another theory that some hold - that Lucifer ruled a planet (possibly called Rahab) where the current asteroid belt is, and that this planet was destroyed by God in judgement for Lucifer's sin. Having lost that planet he was cast down to earth - and then caused trouble for Adam. I am not in any way suggesting this is correct, I am rather throwing it out there as an alternative theory that also seeks to explain Lucifer's fall pre-Adam, but not on earth. I find it at least as compelling (or you could read that "as little compelling") as the idea of a pre-Adamic Luciferian kingdom on earth.
Key supporting verses used:
Ezekiel 28:14-16
"Thou
art the anointed cherub that covereth; and I have set thee
so: thou wast upon the holy mountain of God; thou hast walked up and down in the midst of the
stones of fire.
Thou
wast perfect in thy ways from the day that thou wast created, till iniquity was found in thee.
By the multitude of thy merchandise they have filled the midst of thee with violence, and thou hast sinned: therefore I will cast thee as profane out of the mountain of God: and I will destroy thee, O covering cherub, from the midst of the
stones of fire."
Stones of fire is interpreted to mean "planets".
Psalm 89:10 "Thou hast broken Rahab in pieces, as one that is slain; thou hast scattered thine enemies with thy strong arm."
Isaiah 51:9 "Awake, awake, put on strength, O arm of the LORD; awake, as in the ancient days, in the generations of old.
Art thou not it that hath cut Rahab,
and wounded the dragon?"
Rahab is interpreted to mean the name of Lucifer's planet.
Key supporting physical evidence is the mathematical theory that a planet should exist where the asteroid belt lies today (the planets are spaced in a clear pattern, and there appears to be a gap in the pattern), and the existance of many craters on all other planets that could have been caused by debris from destruction of such a planet.
This theory seeks to explain the same thing, and selected verses can be cited from prophetic and poetic books that appear to support it. Also, there is a clear astronomical theory that supports it. Why not believe Lucifer's realm was here? Why believe it was on the earth?