I utilize the philly shell quite often. You may want to bring your right hand up just a bit, right at your cheek bone. You can pop your head to the inside or outside of your right hand, effectively defending both sides. I like to throw the jab to the body since my lead hand is already low. You can also throw the "up jab" from there, which can be difficult to defend if opponents aren't used to seeing it. Once you can roll away from an opponent's right hand, you already have your own right hand loaded to fire the counter.
From a defensive perspective, there are probably ways to use footwork AND the shell together, but footwork OR the shell works better for me. I bring my hands up to a more traditional stance when I'm moving and like to sit down in the shell and trade. Once you get more comfortable actually rolling, you will have more confidence to stay in the pocket. If you watch Mayweather, he brings his right hand high when he is pulling his head away from an opponents left hand and he tucks his chin to his left shoulder, rolling with the punch of an opponents right hand. If you spar with someone who has that 1-2 cadence to their striking, you can completely shut them down just by using these two techniques. By pulling away and rolling with the punches, it reduces their impact as well.