They're all very solid - for faction wars.
Outside of FW the only one of those I've used is Kaiden, and in extremely limited fashion. But that's largely dependent on your roster - who else are you running? If you have Arbiter, for example, Melga is almost entirely useless.
Outside of FW, trying to figure where i would use them.
Thinking reinbeast and Kaiden would be good for CB but can't think of anything else for them. I don't have Arbiter just yet but working on that now.
right now
AD- Miscreated monster, godseeker, Lysandra, ninja
AO-Ninja, lysandra, bad el, seeker
CB- Ninja, god seeker, runekeeper, MM, lysandra
DT- God seeker, lysandra, MM, Ninja, bad el
Kaiden isn't really worth running for CB. He only brings ATK down, and lots of other champs do that. Reinbeast doesn't even bring that. He's definitely not worth it for CB.
Your CB team has its own share of problems, and I'd suggest maybe starting another thread for that. But for right here - no, I would not bother with any of those four champs. The ones you have are all easily better.
I don't have Cornelia and I am not familiar with her skill kit.
The other 3 heroes you mention - I have used in my Faction War teams.
The Dark Elf only had 2 revivers when I beat Stage 21.
Kaiden or Blind Seer
I used Kaiden because I couldn't get my hands on Blind Seer.
Kaiden help me 3 star a few Stages leading up to Stage 21.
I didn't use Kaiden on Stage 21 though.
The Skinwalkers didn't have revive hero when I beat Stage 21.
Reinbeast was a critical hero because he offered A1 Healing + Shield.
The Dwarf Faction only had 2 revivers when I beat Stage 21.
Maulie or Melga
You really need a Reviver for them because the Boss hits super hard.
He will usually kill your champs.