The center spine on the C D and E. Is it large enough to have crew walk through it, and for that matter is it hollow?

Yes! We're still working out the details, but it should be large enough to walk through on the Hull D and E, and large enough to crawl through (similar to the Jeffrey's Tubes on Star Trek) on the Hull C. In all of these ships, it will be advantageous for an engineer to be able to reach the drive unit on occasion.