My aunt’s estate includes jewelry and statuettes. The heirs disagree on the value to declare. Some want an inventory while I prefer a simple declaration, which is cheaper. Am I allowed?

Yes. Under Article 762 of the General Tax Code, you may make an estimated declaration of the jewelry and statuettes provided the value is correctly assessed.
However, without agreement the estate tax declaration may be late. As heirs are jointly liable for inheritance tax, late interest or penalties will apply to all.