Mike if you went to size 7 1/2 shot and shot the same 5/8 load you would drop your number of pellets from 355 to 211 which should reduce the chance of stray pellets hitting non intended targeted birds. I know after all the shooting lessons given and taken by you and your gang this seems like over kill.
Perhaps you could go in another direction. Start by removing all the pellets and see if your wad is killing the second bird. If not then increase your shell's payload one pellet in the load at a time until you find the ultimate load. Perhaps a shot of your repute needs only one pellet or as you are getting up there in years you need more, like five or six. This should be more fair to the birds and not waste as many pellets with overkill you are experiencing.
Your concern about not killing too many quail is admirable but did you consider what may happen if others had your lease? I have seen too many lease taken by groups who compare number of birds killed with the lengthen of their peckers. Only happy with big kill numbers to brag about their shooting ability and willing to shoot until past dark or until the last bird has been killed on a lease. A good steward, such as your self, should not be too upset about an occasional accidental double. Some claim every bird you kill is the result of an accident or divine intervention, not of the side of the bird. Nasty things some friends say, nasty or envious.
