Will mineral spirits reduce the appearance of lap marks on a stained floor?

I doubt you'd be successful with that approach. It would be very difficult to dissolve and wipe off specific areas and effectively taper it out. That's if the mineral spirits will dissolve cured urethane, which I doubt. Then, you'd have to reapply clear urethane to create a smooth finish.

You may need to sand the floor again and reapply. A product like that must be applied in a single wet application. For more forgiveness, stain and varnish separately. It's fairly hard to mess up a wiping stain, and clear urethane won't usually leave lap marks.