You claimed that 'criminals will never be effected by gun control legislation'. I don't see much evidence of that, especially since gun control laws in other countries as far as I'm aware have successfully reduced general access to guns.
If you are talking about islands like England or even Australia, there's some truth to your words. Gun control laws in both places have reduced access to guns (Which is why the knife murder rate in England has skyrocketed, among other things.). But those are islands, and they still have new guns arriving.
The United States has a land-border with Mexico right at two thousand miles long. Do you seriously think that our porous southern border will be able to keep guns from Central-American Failed and Near-Failed States out? Because they sure as shit can't keep drugs from crossing that border. It's a sieve.
Your argument that criminals don't care about laws seems counter-intuitive to me, since firstly many criminals use firearms that they obtained legally,
This is true about mass shootings, certainly. Mass shootings are not even a tithe of gun crimes. Look at the murder rate in Chicago.
and secondly gun control seems like it would decrease demand for guns, that in turn seems like it would decrease supply, and therefore make it harder to obtain illegal ones.
This looks a lot like victim blaming to me. Because Lawful Gun Owner A had two guns stolen from him, and those guns were subsequently used in other crimes, he is now a part of the problem? This is, however, part of what I mean about criminals not caring about laws. They're quite happy to steal a gun, or buy one out of the back of someone's car.
Where is the evidence that moderate gun control policies would have no effect on the prevalence of firearms among criminals?
Look at Chicago and Washington DC. They have some of the most restrictive gun laws in the USA. They're also its murder capitols. We already have moderate gun control in America. We do not need extreme gun control, which is what is now being pursued. None of the laws currently being advocated would have stopped any of the recent Mass Shootings.
Do you want a second civil war? Because requiring lawful gun owners to sell their guns to the government would start one.
Right, wrong, or indifferent, it would send the message to ~50% Americans that the government was becoming an oppressive European or Soviet-style authoritarian government (more than it already is, anyway).
From the perspective of a large part of the country, the only reason to confiscate lawfully held guns is because the government doesn't want the people to be able to be able to defend themselves against that same government should they continue down the path of Progressivism to its ultimate conclusion: the sort of Stalinistic purges that always seem to occur when a government goes full Left-Wing (Pol Pot in Cambodia, Mao in China, Stalin in the USSR, etc).
Charlton Heston spoke for a large minority of Americans all those years ago when he said, "From my cold, dead hands."
Think I've addressed all your points, but I may have gotten things out of order. Apologies for the spaghetti.