I’m not sure the gunman believed he’d get the shots in that he did. .... he p*ssed his pants which is clear in some of the photos. But I have no doubt he planned this for some time.
As for security - no matter how they react or respond there are always going to be questions. (Heck Reagan and Kennedy both were shot.) This guy went for the head area so I think he assumed in advance Abe would have body protection.
There was obviously a gap in security and Japanese special force experts are saying as such. Not letting suspicious people wander that close to the scene without questioning is pretty much an ironclad rule. Just like so called security details stand around yelling for ambulances and medical professionals several minutes afterwards while trying to lift the body themselves.