In build 52 hostnames looks like "WD TV Live". In build 54 this name looks as "WD_TV_Live". All " " simbols replaced with "_".
Yes, that's correct.
Spaces are invalid in hostnames. Previous builds didn't enforce it.
I think that names here are used for convenience. They aren't used in a network infrastructure. On it I do not see the reasons for such restriction.
You may think so, but they actually are used as hostnames on your network.