If the HA does not agree to remove the sink, then it might really be better to let the HA install the final sinks. Who knows if otherwise ugly holes or similar remain from the dismantling.
None of this is great. Almost every construction project has problems with the sanitary fixtures. It's always the same cliché about the warranty. They want to sell the components at the expensive prices of the sanitary companies, who then also get a piece of the pie. I would accept that if the prices were reasonable, but the markups are usually really outrageous.
At least our first construction company had no problem painting the washbasin and crediting a (even quite fair) amount for it. I don’t understand why the warranty is being argued with you. They just have to install 2 angle valves, end of story. What does that have to do with the warranty of your sink installed on site?