"Why would something so grand contain such a fatal flaw?"
Other than "how else would ANH work" reason, you mean?
Because whoever designed it didn't realize it was a fatal flaw. If you were a software developer (or designer) you'd know that it's impossible to design bugless software of any complexity. A Space Shuttle was destroyed on the account of a piece of foam. Mars-bound lander was lost due to a metric-imperial unit conversion bug. And so on and so forth.
In-universe, it was retconned to be not an uncommon design flaw, . To quote from SW Wiki:
In another example, Ice Station Beta was destroyed by Kyle Katarn when he planted sequencer charges in thermal exhaust ports throughout the facility.
"Would the Rebel Alliance had even attempted the mission if Leia didn't think Luke was the one?"
Leia did NOT think Luke was "the one". Or anything beyond a capable pilot. He was just another X-Wing pilot, not a designated shooter.
He didn't "have the force" up until he took that shot. Leia had no reason to know he had Jedi abilities.