You cannot say with any certainty "these 17 people wouldn't be dead" if the murderer used other means.
The killer walked through a 3 story building over a period of 6 minutes killing 17 people. If he had been in a car, he would not have been able to kill people within the school due to a school being a structure. I can say for certain that a car could not drive into a school building with enough force to kill people on the second and third floor.
we're lucky more kids don't try the bomb route, way more deadly and easier to get/make.
To quote you "you're flat out wrong", I don't think there's more than one case of "kids" building bombs and then using them successfully to blow up students in a school. Columbine they constructed 99 bombs and very small percentage of them went off, and none of them killed anyone.
The Stoneman Douglas killer bought his gun at a local store. How is it easier to buy a fully functioning weapon made by professional craftsmen then it is to buy parts for and learn how to make a bomb?