If what you say was true then once Hashi or Yamato stopped supplying chakra to the wood then it would disappear. This is not the case however. That is why mokuton is so special. It isn't just combining natures. It actually creates plants that can survive and will exist without chakra.
While Nagato is busy blowing the pollen away, that leaves him open for attack. Any counter that Nagato does leaves him open for attack. You also neglect Hashi's apparent ability to heal. Madara said that it was even more effective than Tsunade's. All of that combined with his physical strength proves disastrous against Nagato.
You are underestimating the strength and knowledge of Kurama. You are also speculating that Nagato can create a larger CT than what he had shown to stop Bee, Naruto, and Itachi. We haven't seen this. Even if he could the destructive power of Kurama's TBB would still tear through it or Kurama could just break through himself. Kurama probably has knowledge of the Rinnegan since he was around with the actual So6P.
Nope, it's like any Suiton or Katon based attack, once the chakra supply has stopped, the technique will end but the remnants (fire, water, or in this case, trees) will still remain, and will
still be susceptible to Fūjutsu Kyūin. But obviously, Nagato does not need to worry about the trees which Hashirama is no longer manipulating (just like ninjas don't worry about the left of fire from Katon attacks), so he doesn't need to absorb absolutely everything, just the parts which are of immediate danger to him.
Nagato has shown the ability to use more than one path at a time. He can use Preta path in conjunction to Deva Path and Asura Path if he wanted. Shodaime may have some outrageous healing abilities, but it will not save him from having his soul ripped from his body, which Nagato could achieve with a clever Banshou Tenin (Naruto was only saved by his chakra arm) Human Path combo. If Hashirama is close to Nagato, and Nagato chooses to use Banshou Tenin, Shodaime will simply have no time to counter. And then of course, the option to sync with Gedo Mazo remains. I also feel you're overlooking the effectiveness of Cerberus in this situation. The only way to defeat it is to defeat the summoner, and if Hashirama continues to spam forests, pretty soon there'll be 20 odd dogs running loose, and everytime Hashirama tries to defeat it (or even if it's accidental because of his forest spam), he'll only be creating more problems for himself. Currently, Nagato has wayyy more feats that Shodaime, and this is why,
currently, he is ahead of the 1st Hokage.
You must be registered for see links
Kurama's Bijuu Dama, with the explosion being the size of a single mountain.
You must be registered for see links
the smallest Chibaku Tensei Nagato made, done after fodderizing a village and through Deva Path. You could comfortably house Perfect Susanoo and the two meteors inside that. Just think of how many Bijuu Damas would fit inside of the canyon created by Chibaku Tensei. Hundreds. At the minimum, a healthy Nagato could make one
at least twice the size. Puts things into perspective.