A few more questions. Was the real garden of Eden on earth, or somewhere else?
Yes it was on the earth.
If it was on earth, where exactly was it?
I don't know.
is the location marked in some way?
Vaguely in scripture, but without anyone to take care of it, it ceased to exist.
Lastly, why did Jehovah allow Satan into the garden?
Satan was allowed to offer them an alternate form of rulership so they could freely choose Jehovah's rulership or not. To not have allowed them an alternative would make Jehovah a very different God and would make us very different people. Jehovah knows the alternatives have failed and will fail, but people must come to this realization on their own. All of this time has actually been an educational period for mankind.
Even without Satan the question would inevitably occur anyway. "Why should we listen to Jehovah?" Jehovah is allowing us to find out. Look what occured in ancient Israel.
"4Then all the elders of Israel gathered themselves together, and came to Samuel unto Ramah; 5and they said unto him, Behold, thou art old, and thy sons walk not in thy ways: now make us a king to judge us like all the nations. 6But the thing displeased Samuel, when they said, Give us a king to judge us. And Samuel prayed unto Jehovah. 7And Jehovah said unto Samuel, Hearken unto the voice of the people in all that they say unto thee; for they have not rejected thee, but they have rejected me, that I should not be king over them
." (1 Sameul 8:4-7)
Adam and Eve effectively did the same thing. They rejected Jehovah's kingship over them. Jehovah allowed them to do it. Satan tempted them but the desire was in them. James said, "each person is tempted when they are dragged away by their own evil desire and enticed. Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death." (James 1:14,15)
Satan enticed them but it was their own desire that caused them to eat the fruit.