Hello!
The blockade is for the next form that will be added, for now, that area is un-accessible.
The Membrum core is a very rare item, optionally, if you have the money, you can skip the difficulty of getting it by other means. Also, if you have discovered and resolved the Mandy Mystery, she will sell it for you (If you picked the ending to help her) for a massive discount. Or you can get it as a drop from the Futa poltergeist, which is like a 5% drop. Right now the core has one use, it can be given to your Dryad's house Chota plant to fully evolve it, which has nothing right now, but will be a battle companion, which also includes it's futa scene. Other than that, the Membrum core will be needed in the future to access the Futa Voidseer scenes.