Think about in this way c: You use an upgrade stone if one or more species of your pokemon can no longer evolve. Since Minior can't evolve, but the other species can, you use an upgrade stone.
Same thing with the Absol/Natu/Ralts baby. Absol can't evolve, so the main item you would use to fully evolve this poke is through upgrade stones. And since the Natu part of it would be fully evolved, it can't evolve any further so it would require an additional upgrade stone.