Division According to Court Order

What happens if the testator gave instructions about the division of the estate?

The court will follow the instructions in the will, even if this refers to a property which cannot be divided, unless it is persuaded that there are special reasons for digressing from the instructions.

What happens when the heirs agree on dividing up part of the estate differently to the division prescribed by law?

They have no choice but to apply to court to divide up the whole estate.

Will the court still divide the estate according to law, even if all the heirs oppose such a division?

As a rule , yes, unless they reach unanimous agreement on an alternative division of the estate.