The NP could be partly built in both cities chopping forests outside the fat cross without losing any potential forest preserves inside. I admit that Walata could eventually make a very good NP city but I'm worried it will be slower developing the other infrastructure. That infrastructure will make the NP a lot more effective.
It depends if the OP is planning to run the Caste System or another civic in that category like Slavery or Emancipation. That will depend on the rest of his economy. But it can be hard to get enough specialist slots if a city is founded late in the game as Walata seems to be. Walata could have 15 forest preserves (if I counted correctly) and maybe 2 more forests could grow. Bactra could have 10 forest preserves. Bactra could using whipping easily build a whole lot of building slots to run specialists.
5 Spies : Courthouse, Jail, Int Agency = 120, 120, 180 = 420 hammers
3 Scientists: Library, University, Observatory = 90, 200, 150 = 440 hammers
4 Merchants: Market, Grocer, Bank = 140, 140, 200 = 480 hammers
5 Engineer: Forge, Factory, Assembly Plant = 120, 250, 250 = 620 hammers
1 priest: Temple = 80 hammers
National Epic = 250 hammers (+ 100% marble)
National park = 300 hammers
Total hammers = 2590 hammers for a fully productive city.
It is likely that Bactra can build a lot of those buildings before needing to construct the NP using a combination of slavery and working grassland forest tiles. On the otherhand Walata will have to build more of them after the NP is in place so without running Caste System most the free specialists will be citizens just giving 1 hammer and 3 beakers. In Bactra the lower number of free specialists will give their full normal output AND it will be boosted by the relevant buildings (e.g. merchants by banks) and of course their GPPs will be boosted by the NE.
It is a simple case of more forests in the NP city does not necessarilly mean more output over the course of the game. You have to consider the time it takes to build everything needed to make it really work well.