What exactly is causing the insurance concern: the premium, restricted cover, or uncertainty that suitable cover can be obtained at all? Those lead to different buyer reactions. A known cost can be reflected in the offer, while unresolved availability may make a buyer—especially a financed...
The practical offer case should probably be property-specific rather than “58 days equals X% off.” Use defects, renovation cost, uncertain possession, repeated cuts and genuine nearby alternatives to support the figure. If the condo is vacant, well presented and the seller is not under pressure...
I would also keep purchase costs separate from new-build handover costs. Snagging, connection or activation charges, changes requested from the developer and fitting out the flat might not appear in a legal closing estimate at all.