Not a lawyer, but I understand the concept of precedent. Do you really want to set the precedent of severely restricting rights on the Second Amendment then be in denial about the impact of precedent it has on all other rights?
We already restrict rights all over the place. I have the right to peaceably assemble, but not in the Oval Office. I have the right to free speech, but I'm still subject to noise ordinances. There's freedom of the press, but the government can still force me to pay someone else money if I use that freedom to libel someone. My freedom of religion doesn't, for example, give me the right to murder witches, even though the Bible very clearly orders me not to suffer a witch to live. I have a right generally not to be searched without a warrant, but if I want to fly on a commercial airline or visit a secure government building, or all sorts of other daily things, I'm going to have to submit to a search without a warrant.
Press freedom doesn't mean you can broadcast anything you want on any frequency you want. The advent of radio communications created a bunch of issues that were different in key ways from what the founders were thinking of when they spoke about a free press, and so we have FCC regulations limiting what you can do. Similarly, the advent of semi-automatic weapons created a bunch of new issues, and new regulations make sense.
As for the Niemoller, the argument wasn't that if you allow one government regulation, then it's a slippery slow to totalitarianism. In fact, the first people they came for were the communists, who were pushing for more government regulation. Niemoller was talking about attacks on individuals, not regulations about what you can buy.
Yes, nutjobs with guns are a problem. Gang-bangers with guns are a problem in Big Blue City shitholes.
The liberal cities in liberal states tend to be in pretty good shape, and that's partly due to gun control at the state level. NYC, Boston, Seattle, San Francisco, etc., all have lower murder rates. They're not as peaceful as big cities that are in COUNTRIES with tighter gun control (London, Berlin, Tokyo), but they're not bad. Sadly, there are also big cities in red states, where gun regulations are a whole lot looser, and they tend to be shitholes. Memphis, KC, St. Louis, New Orleans, etc., are all plagued by guns.
IMO, it's smarter to work toward saving the 46K+ annual suicides each year....
Gun control can definitely help with that. It's no coincidence that most of the high-suicide states have high gun ownership. Having a gun conveniently at hand means depression has to only be bad enough to drive self harm for an instant, and the person will be dead. In a place where guns aren't conveniently at hand, there's a lot more chance for a person to come to his senses, and a mere moment of despair isn't going to do it.
If we want to focus on suicides, a good start would be figuring out what's going wrong in shitholes like Wyoming, Alaska, and Montana (which lead the nation in suicide rates), and what's going right in happier places like NJ, NY, and MA (the lowest suicide rates). Since I take it you aren't interested in coming at the issue through the obvious method of gun control, what other ways would you work on to make WY, AK, and MT more like NJ, NY, and MA? How do we clean up the shitholes?