Nice design, the concept seems to work pretty well on Pete's antweights so it'll be interesting to see how it does on a feather. A couple of things to mention that may help your design/build:

Presumably due to eBay seller fees, the actuator you linked to is about £10 cheaper if you order it directly from Gimson Robotics: http://www.ebay.ie/itm/High-Force-Co...PiB0PiTCtG0bcg

My personal feelings are that the top spines aren't large enough. The front lifting arm/plate is quite long and looks quite steep even though it's 30°. Featherweight fights are quite fast paced so what I expect might happen is that as you get under an opponent and start to lift them, they won't be far enough up the arm to be snared by the spines so may well just slip off or get turned over. If you attack a wide robot, the side tabs on your arm look like they'd stop the opponent from reaching the spines, leaving you unable to grab them as you lift. What is the distance from the tip of the spines to the lifter plate? It looks quite small though hard to tell from the CAD; I'd say the average height of featherweights is about 100mm, so ideally being able to accommodate that would be a good shout.

A 5s pack should be fine and not cause a problem for your motors. The general rule of thumb for overvolting is 50% above the rated voltage, so 12V motors on 18V (or 18.5V in this case) should be fine. It'll give your actuator a bit of a boost too, as the load speed at 12V is only 5mm/s. I don't know the geometry of your arm to work out how fast that will be at the tip but, again with reference to the fast nature of featherweight fights, it might seem a bit sluggish or make it difficult to get a proper attack on an opponent.

Just my two pence