The intelligence failure, where Israel did not recognize Hamas true intentions and capabilities over the last 20 years, led to a serious failure of basic security precautions.
FTFY
Israels intelligence services decided that Hamas was neutered, defeated, and no longer interested in war with Israel, rather being in control and building some sort of kleptocracy in Gaza.
Any threat of infiltration was judged to be a small cell of terrorists. They never, ever dreamed of a full invasion by massive numbers of terrorists.
Which explains all your questions.
But for Gaza, Israel had the military power, if not the diplomatic cover, to neutralize Hamas long ago. And the cost of keeping a much larger force to watch Gaza, canceling leaves at the first sign of trouble, or keeping more helicopters and fighter jets ready is so small. And when you hear that a few smaller places were spared due to a simple locked gate, you wonder why
weren't all the kibbutzim within walking distance of Gaza property prepared?! Why were the local security under-equipped?! Who let a festival like Nova go ahead in such a place and with whatever security it had? Why weren't real plans drawn up for defense against an invasion?
The enemy is not interested in war, they want peace. You don't attack that enemy, you live with them and encourage them to be more peaceful.
They spent billions of shekel to create a system that would allow them to watch the Gaza border very well and deal with any sort of infiltration.
Canceling leaves is on of the failures of the night of Simchas Torah.
The IAFs readiness costs more than you would believe, and was affected by all the shtuyot of the previous year.
The places that were spared were Yad Hashem. Locked gates were easily breached in many yishuvim.
If there was no invasion threat from Gaza, then the yishuvim WERE properly prepared for infiltrations of lone or small cells of terrorists.
Why would you not approve Nova in these conditions?
And due to all the intelligence, analysis, and imagination failures, they never dreamed or conceived of the possibility of invasion, so they never even thought of the need to consider drawing up plans of defense.