Trait rolling matters so much in Anime Defenders because a strong Trait can more than double a unit's damage output, and it doesn't even unlock until Level 20, leaving units traitless and at a real disadvantage before that point.

No, you can't trade a unit that has a Divine Trait, since Divine Trait Crystals themselves are non-tradeable and units carrying Apex or Divine traits are restricted from trading for the same reason.

No, not all units evolve the same way, since every unit needs its own specific crafted item alongside Star Rifts, and only a small number of Mythic units currently support Double Evolution on top of that.

Yes, you keep the units, Trait Crystals, and materials included with my purchase, exactly as shown in the listing.