Capacitance

1. if K < 1 then does that mean the dielectric will decrease capacitance, or
does that mean it's just not a very good dielectric?
In theory, that would be true.  However, all dielectrics have a K>1 value, because all dielectrics are designed to increase capacitance.

2. the explanation for #9 in physical sciences on full-length 1 includes
this chunk: "the capacitor is in series with the transformer and therefore
charging" -- I guess I don't understand why that is, and what if it was in
parallel?
For a capacitor to charge, it must be connected to a voltage source.  Here, the transformer is serving that function.  Given the circuit setup, there's no way for the capacitor to be in parallel with the voltage source, since that's the only thing it's hooked up to.  Either way, a capacitor will always be in series with the voltage source, since that's where it draws its energy from.

3. I also just didn't understand the explanation for 18 in physical sciences
on full length 1.
Writing out the explanation for this question is very complicated; I'd encourage you to watch the Explanation-on-Demand in the test, since it gives a very good explanation of all the math here.  In general, I think Explanations-on-Demand do a great job explaining the science questions (plus, you get to hear my lovely voice some more!)