Nissan have a strategic partnership with Renault, I doubt they would enter Formula One if Renault are still around.
True that, the various big companies eating the smaller ones mean that some of those I think of as different are not. What do we have:
Ford is also partlyMazda, Lincoln, Jinangling
VW is also partly AUDI, Bently, Bugatti, Lamborghini, Porche, SEAT, Skoda
FIAT is also partly Ferrari, Chrysler, Maserati, Alfa Romeo, Lancia, Dodge, RAM, Jeep
Renault is also partly Dacia, Nissan
Peugeot is also partly Citroen
Citroen is also partly Peugeot
Tata is also partly Jaguar
BMW is also partly Rolls Royce
Guess we in theory should disregard a lot of those, however unless they start working out of existing engine shops already active in F1 then I will see them as 'new engine manufacturers' (except those who have been there before).