The snakeish thing doesn't make sense seeing as they have to keep some continuity between the films don't they.
I think they are going to do it like they did it in CoS and GoF. It will look like there are 7 Potter's otherwise that scene wouldn't work and for the Gringotts robbery - its only Hermione as Bellatrix and they'll do some wigs etc for Ron, Ministry - they'll look like who they are trying to be and godric's hollow - maybe just forget polyjuice and just go under the invisibility cloak. To me, i wouldn't like it if we the audience saw them as themselves coz to me that takes the magic of those scenes away/