Welcome to the forum 
1) Can I wire up additional drums via a ¼” Midi patch bay?
No. You can buy a second module (I use a td6v) or a TMC-6 to do this.
2) Has anyone tried using a Double Kick pedal with the KD-8?
Yes, of course. No problems.
3) And finally, the kit came with three PD-8’s. I would eventually like to buy two larger floor toms and only use 2 (of the 3) PD-8’s. Does anyone know if I could use the extra PD-8 as a Kick trigger?
I would look to replace all the pd8s eventually, your wrists will thank you. If you can figure out how to suspend the pd8, you can probably use it as a kick trigger - other people have used mesh pads as ersatz kick triggers. Would this be instead of the kd8 double kick or in addition to???
TD9+6v module, KD-8, custom Diamond Electronic 12" snare + 2x8" + 2x10" toms, PD-85, 4xPD8, 2xCY8, CY-5, CY-12r/c, Pearl P902 double pedals, Diamond Electronic COWBELL!
HPD-10 ATH-50M phones and 4 Vex Packs for the TD-9
Gibson Les Paul. + pod xtl, Norman acoustic, Fender J-Bass + gt10b
Clavia Nord Stage 2 Synth/Stage Piano Samson SR850 'phones
cubase, garritan personal orchestra, JABB, symphonic choirs, Sibelius, reason 6.5, IPAD 2 with lots of soft synths...
three shelter cats