Oh I did discover something that I'm hoping might help!
I downloaded a feed reader to look at both feeds generated by the Podcast Channels. The one for the default (original G2V) show is reading just as it should - 24 entries, just the podcast posts, no blog posts, nothing extraneous. Looks fine.
The feed generated by the new channel for the second show, Doctor of the Dead, when viewed in the reader, is showing *everything* - the Doctor podcast posts, the G2V podcast posts, and even blog posts and other non-podcast posts from the site. Not sure why that is when the channel was created as instructed and all the fields are filled in correctly (two or three times in some cases), but clearly something is very awry there. For some reason, the new channel is a feed that is not podcast-only and not Doctor-only - it's the whole site.