If you want I can email you the structural engineers calculations when I had our through lounge created 3 years ago at the same time as our loft conversion. You could then email that to the buyer together with a photo of the size of my steel beam and that might give them some reassurance that the size of the beam is either adequate or will need replacing. Either way it would mean you don't need your plastering hacked away at and they know whether it needs to be replaced or not.
If you want I can email you the structural engineers calculations when I had our through lounge created 3 years ago at the same time as our loft conversion. You could then email that to the buyer together with a photo of the size of my steel beam and that might give them some reassurance that the size of the beam is either adequate or will need replacing. Either way it would mean you don't need your plastering hacked away at and they know whether it needs to be replaced or not.