Decoding audio should be generically much, much simpler than decoding video, so ffdshow or something else for audio doesn't really matter (unless there are some other problems, not performance-related).
JRiver media splitter - wait, what??
By the fact that you're trying to eek out 720p playback out of your laptop I take is not a recent unit. EVR was invented for Vista and later and so it requires some GPU power which may not be available (meaning a GPU with some power not the integrated units from 5 years back). So, NOT that (even if you can get EVR in XP). The fastest (and most limited) video renderer is Overlay.
CoreAVC decodes MPEG-4/H264/AVC ONLY. DVDs are MPEG-2. So no.