I did not see a justification for either taxes or conscription in that chapter, But I can see where you might have. He states, quite correctly, that if one were to to take libertarian principles and blow them out of proportion, the results would be quite silly. The examples of turning on one's lights, or breathing into the atmosphere are good examples. Examples to which I have a workable answer: Harm. If harm comes to you, then I am liable for repairing those damages. Obviously, shining a flashlight at your door would cause little or no harm. A high-powered laser, however, would cause significant harm.
As to the "huge invading army needs opposing huge army, and thus conscription," No, no it really doesn't. First off, the huge invading army will trigger a response from the defense agencies, some of whose clients may have had discounts for agreeing to fight with them should something like this occur. So we have a militia. Add to that the fact that unless they are pacifists, each home will be defended by the home owner. Finally, add the fact that with no laws against their ownership, private citizens can, and probably will, own any weapon available, up to and including nukes, in some instances. Those add up to this simple conclusion: Invading an ungoverned area will be costly in both funding and lives, not to mention time, since there is no central power to take over. If an invading army took DC, both sides would probably count the war as over. With no capitol to take, the territory needs to be taken house by house. Imagine playing a game of chess where one side had no king. Winning that game would be possible, but expensive, and long-fought.
Read it again. First he deals with the NAP, then the rifle example is a justification for taxation and then he justifies the draft. Your idea about resistance is dealt with.