I'm definitely on the "either Hanoi or Tahiti is only one leg, or we finally have 13 legs again" bandwagon too.
Even if it's Scotland instead of England, there's no way on Earth the government of Northern Ireland lets the producers in if they're planning on spending half of the episode in the rest of the UK. So if those two are both true, they're separate legs.
Maun is too much of a dogleg in the route to spend only one leg in the region. To put it into context, going from Hanoi to Maun to (probably) Zurich in two legs is roughly the same as going from Okinawa to Baghdad via New York. It doesn't really matter whether the other leg down there is in Johannesburg or Gaborone or Windhoek or Livingstone or Harare, but combined with the show's budget there has to be another leg in southern Africa to make it worthwhile.
So that's four legs (at least). Switzerland and Berlin are definitely separate, as are Christchurch and Bali for eight. Add the US finale makes nine, and we have to somehow fit the four legs currently allotted to Hanoi and Tahiti into three. My guess is Tahiti is only one leg due to cost (seriously, that island is all-caps EXPENSIVE), probably with a 'camp on the streets' opening night bunch/separation like the old seasons, and we're getting two in Hanoi (which is officially the cheapest city in Asia for backpackers to visit at the moment).