I think the grapes have to have been worked on seperately as you say, it would have been almost impossible to do from the outside, especially the leaf at the top. The bunch of grapes reminds me very much of the thin walled glass christmas ornaments, which is why I was thinking of Lauscha.
It is hard to see in the photos, but there is a hole in the base of the decanter, so the grapes form part of the base, with the hole being the inside of the grapes, rather than them being a separate piece added on - it is hard to describe too!
The technique you describe would work, but there is no sign of a join between the grapes and the base, it really looks as though it was made in one piece. You have made me consider whether the grapes and base were made together, then the walls of the decanter joined onto the base. That would be more plausible looking at it.