Regarding the Mercury recordings, I do know that the three-mic recording technique goes back to Stokowski days and was considered a very good sounding technique for balancing detail from the orchestra with the acoustic of the hall. I note that the BBC still uses this mic array for concert broadcasts (you can see it at the Proms in the Albert Hall, three mics dangling from the ceiling). I guess the proof is in the pudding and the Mercury recordings do sound very good.