The main reason Jiang Yi had entered seclusion was to refine the Smelting Refine Furnace, but also because he didn't want to see Yin Ruobing and Yi Chan. Otherwise, he wouldn't have emphasized to the maidservant that he wasn't going to see anyone.
After half a month in seclusion, his injuries were roughly recovered. All his bones had healed up, and his internal organs were much better. At least, there wouldn't be any problem for him to execute the Heaven Evasion.
She Fei's demonic blood was poisonous, but he didn't consume any antidote elixir. After the Destitute Dragon Grass had reformed his body, his recuperation ability was extremely powerful; and he was immune to nearly all poison and would automatically be able to neutralize it.