I cannot answer you question directly, but as per info in the 1st paragraph I would like t say something.
"I have parts in my design, which are not present in the actual later circuit, but which I need for the simulation."
As one of the design rules, I have learnt up to now to keep everything which is not intended to be synthesized in the test-bench. It simplifies the steps during design, simulation and synthesis.
So you might try this design approach so as not face the problem described in the 2nd paragraph.