Yes, antimissile systems will shoot down most of your missile volley, so you need to launch enough that they become overwhelmed and the few that make it through accomplish your goal.
We don't know exactly how much "most" is, but its enough that the powers that be consider our current level of armament to be necessary.
So the way it looks now, Trump has won the presidency, and his allies will have the senate and house of representatives, and they already had the supreme court. The three branches of government will not be working as checks on each other's power, unless we get very lucky and the various factions that make up the GOP split. This is obviously very, very bad, but there are still some checks on presidential power.
Trump's last term was a clusterfuck. Things may just be so disorganized that he struggles to actually get what he wants done.
The states have limited power to defy the feds. While case law does state that federal law supercedes state law, that doesn't mean all States will immediately cooperate wholeheartedly. Obviously a court battle will eventually get to the supreme court, but that takes time and requires a single panel of judges to beat multiple states into line on each new policy.
Governments do have a small amount of caution when it comes to their people. One thing the crazy conservatives had right this whole time was that fundamentally, nobody was ever going to come for their guns because nobody wants to force a confrontation with a bunch of armed lunatics. In the same way, they'll probably try to avoid massive riots and general strikes simply because it isn't worth the fight to whoever is responsible.
Citizens can resist. Go to protests, donate to political advocacy organizations (the ACLU will have its work cut out for it), and for Christ's sake, go vote! Show up every year, just not every 4 years. Without the cooperation of congress, his power would be significantly curtailed.
If nothing else, terms are limited. In 2 years we can swing congress. He isn't going to be able to pass a constitutional amendment to do what he likes before that. If we swing congress in two years, it will slow him down significantly, and then we can replace him in 2028. Hopefully people will actually keep showing up long enough after that to reverse all the damage he's likely to do in the next 4 years.