Someone else on this blog just asked a question very similar to this, so I will try to explain. First off, your polygraph experience is rather typical. Polygraphers accuse a lot of people of withholding information. They do this because they are trying to get you to confess to things that a background investigation would not be able to reveal.
To answer your first question, if you have been told that you will be taking a second poly, that is a good sign. If they thought you were not suitable they have not scheduled that second poly and you would be notified that you were discontinued in the process.
To answer your second question, it might be a problem that you forgot to mention your drug use on your SF-86. The polygrapher may try to spin this as you were trying to be dishonest during your onboarding process.
To answer your third question, my advice moving forward is to remember that the polygraph is not a mind reading device. Calm down, relax, and don’t be intimidated by the polygrapher’s empty threats. They are trying to get you to confess to anything. Therefore, DO NOT make any false confessions to try appeasing your polygrapher.