it was possible they were destroying earthforces military infrastructure all the way to earth then heading back out to the outer colonies to finish the extermination of humankind
That's exactly what they
were doing, as described in
ItB - by-passing civilian colonies, wiping out military forces and installations
on the way to Earth. The idea seems to have been to degrade the Human defense capabilities and keep them under pressure, either destroyed or cut off from Earth and incapable of assisting, rather than waste time seizing and securing territory, which would presumably buy time for Earth science ot develop new weapons and increase the defenses in the Sol system. Compare MacArthur's island-hopping campagin in the Pacific or Grant's constant flanking operations from the Wilderness to Petersberg during the Civil War. Both made the enemy's military forces, not cities or territory, their target.
But the assumption was that they would follow this plan
to Earth, which they would then wipe out, exterminating the rest of mankind at their leisure on their way back to Minbar. There is nothing to suggest that they would wipe out everything up to and including a raid into the Jupiter-Mars asteroid belt, then turn around, exterminate more civilians, and then come back two years later to finish Earth. (In fact Human casualties were failry low, all things considered, precisely because the Minbari never got around to the "slaughter the surviving civilians" part of the plan.)
The main jump point in the Solar System was at Io I believe
Just to claifiy, the main jump
gate in the Sol system was at the transfer point off Io. But Minbari cruisers did not rely on jump gates. At the Battle of the Line they are shown jumping directly in from hyperspace by forming their own jump points. In season 4 Sheridan's forces form jump points quite close to Mars (the
White Stars within the Martian atmosphere itself) and later everybody jumps right from Mars and emerges in Earth orbit. So the Minbari didn't
have to emerge where they did. Chances are they chose that location because Earth's defenders were and they wanted to wipe them out before engaging the orbital defenses. It also gave them more time to learn about those final defenses by sending in scouts and probes. (This is one reason the Grey Council decides to interrogate the few surviving fighter pilots from the Line, after all.)
Had they by-passed the Line, the Minbari would have risked being attacked from the rear or in the flank while dealing with the oribital platforms, trapped between hostile fire from below and above, with little room to maneuver due to Earth's gravity well. (Lefcourt fears being put in just such a position in the battle above Mars during the Earth Civil War.) So it made sense for several reasons to enter normal space where the main enemy force was clustered rather than closer to the final objective.
Regards,
Joe