Can you use Durnevhiir outside of the Soul Cairn? What's the difference?

Yes and he brings undead.

basicly durnevhiir likes to bring some of his friends with him

Durneviir can be killed in 1 shot by another dragon before he takes off...other than that they re just summoned differently and do different shouts

Their is no difference, they both svck an equal amount of svcketry and non straightness.

For example, Odahviing can be summoned by shouting its name to the sky, while Durnehviir to the ground (more specifically when the crosshair isn't red). Also Durnehviir will be able to summon undead creatures from the Soul Cairn. Though, despite Odahviing's shout, in order to succesfuly summon Durnehviir will require all 3 words to be shouted, not only 1 or 2.

Neither of them svck, in my opinion. They whoop ass when I summon them. I like Dirnehviir because his summons attract aggro, while he attacks from the skys, and Odahviing, because he seems to grab enemies and drop them, and go into ground combat a lot more. Also, Dirnehviir is summoned at your feet, so u dont need to wait for him to fly in, so that is nice. They by no means svck, unless they're fighting dragons, but thats because dragons fly around too much and blow weak fire attacks at each other instead of going into close quarters combat.

Not sure what u mean since u always have to say all three words to call both dragons.

Durnevhiir can be summoned immediately, unlike Odahviing, who you have to wait on after calling.
He uses frost breath whilst Odahviing uses fire.
He summons undead minions.

They are both very similar so it really just depends on who you prefer more.

You're right.

Odahviing takes a moment to appear when you shout for him, but he is physically stronger and will breathe fire fairly often. He will also stay with you until there are no enemies nearby.

Durnehviir does not do much damage, but can and will use the Drain Vitality and Soul Tear shouts, as well as summoning undead to fight with you. He will stay with you even if there are no enemies around, but he has a time limit. He appears instantly when shouted for though, since it's basically a conjuration shout.

Then you have not used them. During the Forsworn Conspiacy quest, I was being chased by roughly 25 guards, so I ran out of Markarth into the open field in front of the stables and called Odahviing. He roared, landed in the middle of them, and went crazy. Six of them broke away and came after me, and by the time I had dealt with those, he was casually tossing the last of his enemies to the side.

Both of them are very powerful, and are most useful when you are being attacked by a large group in an open area.

Odahving level caps at whatever level you first meet him at, being equivalent to an Ancient Dragon if you meet him at Lvl 50+. I expect Durnehvir does the same, but what i'm wondering, is if since they added new Revered and Legendary Dragons, does he level up to their potential?