Does the Unicap Liberty 14 AWG/2 Pair disconnect from the center channel speaker? What are the size (gauge AWG) of the cables going to the Front speakers?

A simple solution may be to use thinner cables that fit the spring clips and wire these to both center channel speakers, and take out the 14 AWG cables.

If you want to keep the existing center channel cables, one possible solution is to use the electrical wire nuts (like that used for household electrical wiring) to combine the two positive wires and a third, short piece of insulated wire. Do the same for the two negative wires and another short piece of insulated wire. The third wire goes to the spring clips, with wire sizes chosen to fit into the holes on those clips on the AVR. Make sure no bare wires are exposed from the wire nuts or the spring clips.

Note that the wire nuts is a quick solution, and may not be the best 'sounding' from an audiophile perspective. I've never used wire nuts in an audio application. If it were me, I would twist all three wires together, solder all three and use shrink tubing around the wires to electically insulate it. If you do not know how to solder, get someone who knows how to help.