No that theory is waay of IMO. The juugo clan has no importance like that. Kishi once said in an interview that he only gave Sasuke's eam members back-grounds cause the "managers" wanted it.
This aside, it's still way off. Mokuton has nothing to do with natural energy. Look at Yamato... He clearly explained, how he could use mokuton. To be frankly, the reason seems as simple, as Hashirama being the only known senju with water and earth release U_U
If you, however, are convinced mokuton is connected to his sage mode, well it can be explained more simply by looking at Naruto or Jiraya. They use toads, and summon them freely. However, what if Hashi's SM is the woods itself? That would explain, why he can use mokuton freely, and it would explain, how he obtained SM.
Using wood release as a ninjutsu has no relevance with natural energy. You are mixing things up. There no "special" jutsu accompanied with senjutsu. Senjutsu merely boosts your abilities. Surely Naruto could use it in a way to enhance or change simple elements like rasen shurinken or a simple punch loaded with senjutsu force, however, in the end it has nothing to do with new jutsu. Jiraya neither got new jutsu from it, his jutsu was just enhanced as well.
Which is the main reason, why I say, your theory is off tracks. Besides let wait for next chapter, I have a feeling that Hashirama's father could use mokuton. He's the one resembling Hashi the most.