The important point is that the Corleones were not supposed to know the location of the meeting. The answers to your questions start to become clear if you keep this in mind.
Solozzo was (rightly) afraid that the Corleones would try to kill him at the meeting, so he took multiple precautions to ensure that Michael was the only one there, and that he would remain unarmed.
Michael’s lack of involvement in the family business up to that point was a reason for both sides to feel safe. The Corleones are reasonably confident that Solozzo won’t harm a civilian, and Solozzo doesn’t view Michael personally as too much of a threat.
The location of the meeting was a tightly held secret. Solozzo is not aware that the Corleones found out the location, so he has no reason to go anywhere else once he has Michael in the car.
The weaknesses in Solozzo’s plans were his misjudgment of Michael and his use of McCluskey. As a police commander in an age before cell phones, McCluskey had to let people know where he could be reached in case of an emergency. The Corleones had lower level cops in McCluskey’s precinct working for them, so they were able to find out where McCluskey said he would be that evening, and correctly infer that that’s where the meeting would be held.