Few thoughts on this.
First, 16 gauge probably isn't big enough for what you want to do. I always go big on wire, because
(1) I don't want to burn up my jeep
(2) I may want to add stuff later and don't want to have to re-run the wire, and
(3) I don't want to burn up my jeep
If you really need 15A, you should run at least 14 gauge, but just go for 12 gauge. Better safe than sorry. Google "wire size chart" and have a look. Then go up one size just to be safe.
Second, I had a couple things wired up at the rear of my JK where I shared a ground. One of them was a camera that fed into a rear view monitor up in the cab. The other was a low-draw rigid LED light. I ended up frying my camera and monitor due to voltage spikes, that I learned was most likely caused by a shared ground wire. I didn't investigate this much further, so don't know exactly what happened, but I never ever share ground wires anymore.
If you want to put a power distribution block under the seat, run both a power and ground wire from your sPod into the cab and to your bracket. Mount something like a Blue Sea fuse block like this
and fuse each device separately. I have this exact setup with my sPod and it works great.