"**how do I get it to taper with a broad base without it creating a mirror spiral above the bottom spiral?**"

don't dial the taper morph too much above +1.00/-1.00 in the parameters, I didn't set a limit on the morph and it was created at the extreme of the taper.

"I would like it if the end top coil didn't have to end in a point but be open (like this: https://s3.amazonaws.com/img.charteo.com/art_pictures/C0048/3D-Spiral-Charts_C0048_002_c01_l.png)"

Fiddle with scaling and taper, utilize transparency on the materials (there are 16 material segments to the spiral) to "crop" the coil. see illustration following on page 2 of the thread.