Hi! Sorry for the late reply, I had a really bad internet connection here yesterday but anyway;
First: unsourced work
I reblog the post and add the source in the caption: the title/name of the work and the artist, and links leading to both the work and the artist. HOWEVER, that is only if the artist doesn't mention anything about 'no reposting allowed' or anything along the lines. If the artist doesn't wish for their work to be reposted, I don't reblog the post containing their work, but message the reposter to take it down instead.
Second: if the source is deleted
If the artist deleted the work, I just write "the artist deleted the work" then add the artist's name and a link leading to their site as per normal.
If the artist deleted their account, I write "the artist deleted the work and the account" but include the name of the artist nevertheless. That way people can know who the artist is even though there is no link leading to the profile of the artist's.
As such, your caption works fine too!
Third: messaging the reposter/reblogger
Call me hypocrite, but I message only the reposter and not the reblogger(s). There are countless posts out there that contain works that the artists have explicitly written on their profile to not repost/redistribute/etc, and there are countless people who have reblogged said posts with and/or without awareness of the artists' wish. That said, there's no way I can message ALL the rebloggers to take down the post, no matter how patient I could be what with the ask limits and the length of time it will take for me to message each and every one of the rebloggers.
As for the template, it's the least I can do to help prevent people from reposting without the proper etiquette, so there's no need to thank me ^^
