The Reshiram you get in White 2 is N's Reshiram, so does that mean that it's always the same? Because I want to make it modest by using a modest synchronize Pokemon at the front of my party. Is it possible to make it modest? And at what point do I first need the synchronize Pokemon at the front of my party, because I remember you battle him using his Reshiram before you catch it? (I'm playing through White 2 again and I'm almost at this point).
Yes, Synchronize will work on Reshiram (the only Pokemon it won't work on in B2/W2 are Pokemon revived from fossils, and ones from Entree Forest). Source
As for the second part of your question... You only need to use the Pokemon with Synchronize when you encounter Reshiram as a wild Pokemon, since the game doesn't treat the one you encounter as the same exact Pokemon as N's Reshiram.
Synchronize Works 50% of the time BUT only when the Pokemon with Synchronize is out on the field