To answer your question about why the shield doesn't match the splash art I'll have to explain basic 3D modelling as best as I can (I'm not a pro in the sphere myself).
3D models are created using the so-called "polygons". Each of these is made up of a dynamic number of triangles, called "faces". The most basic shapes are created using just two faces for each polygon, but the more complex the shape is (such as Pantheon's shield) the higher number of faces and polygons you'll need to construct it. If you want to make the shape as smooth as possible, then you gonna need a very high polygon count and, obviously, the higher the polygon count - the more GPU (graphics card) and CPU power you'll need to use that model in-game.
How does that tie into Pantheon's shield? Older computers were slower and had a much lower "render complexity ceiling" (I'm putting this term into quotes because I'm not sure it exists among graphic designers and ITs, I just made it on the fly) than today's computers. Therefore, to let your game run smoothly on the vast majority of PCs you'd need to use models that are as low-poly (-poly being an abbreviation for polygons) as possible and as little/least detailed visual effects as you can without making it look absolutely ugly. It's basically achieving the perfect balance between quality and quantity.
It's also the reason why in the Awaken cinematic each character's model (especially Draven's because of his beard and the fur on his outfit) probably has a polygon count literally in the seven digit numbers, i.e.
1 000 000+ polygons, in order for them to look so incredibly detailed. The in-game graphics could all be brought to that level, but not only is this probably going to take years to achieve, how many players do you think possess gaming PCs capable of rendering this level of details in real time at even 30 FPS, let alone 60 or more?