For some reason Shadowmere never appeared when I went through that quest stage in the DB quest. Is there any way to get him to appear through a console code?

She's been vanishing on me occasionally ever since I got Dawnguard. Luckily the PC fix is simple:

prid 0009CCD8

moveto player

He tends to get stuck at Castle Volkhair if you fast travel there and back. If you fast travel to the dock and ride the ferry there, this problem doesn't occur.