Concerning Dragon stones

Nothing in Fire Emblem is less understand than the dragon stones, it’s gotten to a point where even some of the writers don’t even know their own lore. 

To start contrary to what you might have learned from Fire Emblem engage nether a divine dragon or fell dragon require a dragon stones to take the form of a dragon that is something they can do naturally as seen with Corrin in chapter 5: Mother takes on his dragon from before getting his/her dragon stone. 

The way a dragon stone works is as a limiter on the dragons powers so they must use their full strength without being overwhelmed by like the children of the first dragons who’s powers drove them insane and they become like terrible monsters.