This is a common misconception of our ROOTS models.
The screws "Part C" are supposed to be loose - and it is not possible to tighten them.
The "loose" screw will provide a certain parts of the sofas flexibility and avoid too much pressure on the wooden parts. However this does not make the sofa or wooden parts defective - the functionality will not be affected by that.
Replacement of spare part will not help the issue.
Replacing the part will result in the same situation.
We would recommend that you finish assembling the sofa and thus you will find that the sofa is stable and you will not notice the extra long screws.
Please see the pictures.
I you have any questions about the assembly and construction of the product, please reach out to our customer support team.