RE: seamless glass: I did a relatively indepth post about the trouble with glass about, ooh, two years ago now, and the situation has not really changed since then:
TL;DR: We don’t support semi-transparent block textures, which makes nice glass very difficult to do. I am not inclined to make a ‘seamless glass’ block that is completely transparent but still solid because that seems like great fun for griefers and not much use for anyone else.
Making glass panes as meshes that you could combine in the same way that signs work is likely the best solution for this issue, and window meshes are definitely down on the list of things we’d like to do, but it’s not currently in development.