Sounds like the plan was a grenade that failed to open the door.
Where did you read about a grenade exploding at the door? I have not read anywhere yet that they fired a shot before getting to their primary target, the 2nd floor office.
From what I have gleaned, the plan depended on a silent entry. The “weak link” was letting a mom with a kid know the security code. They may have watched her return from lunch every day. If so, they get an F- for their security plan, considering they had already been bombed, and were a major terrorist target.