spread it. You're not supposed to do that to aluminum, but the thing is a cruiser. It's not like you're taking that thing over stair gaps.
Or you could just find thinner axle spacer for your shimano hub. You might be able to shrink it 11mm, but if not at least you won't have to spread the frame as much.

it looks like the A2Z adapter is the only viable option for fitting discs as the thing doesn't have canti bosses, but it does have some humongous dropouts.

I would just ride the fleetwood. Unridden bikes make me sad, and bare aluminum frames don't make the ladies weak at the knees.