1) Let's assume that you're correct. We're talking an increase of high 30's to high 50's. Per year.
In a country with 20,000 odd firearm homicides a year.
So the assault weapon ban saves 30 lives a year, at the cost of restricting the behaviour of tens of millions of law abiding citizens. Interesting authoritarian bent you have there.
I mean, next you'll be arguing about how we should ban cars because some people drink drive. Just so so silly

2) Doesn't appear that the ban made any difference to the trend in violence. Correlation doesn't equal causation, sorry.

2a) corollary - this is mass shooting deaths total, ie, it includes mass shootings with handguns. To really draw any type of conclusion from this data set you have to control for firearm type, which, they don't. Even then, it is a meaninglessly small problem in terms of the greater US gun violence problem.
If you actually knew anything about this topic you would know
most gun deaths in the US are due to hand guns, and in particular, handguns that are either carried illegally/inappropriately or entirely unregistered.
You have to ask yourself the question "why then is there such a focus on banning semi-automatic rifles". It's a conundrum!