You  are  badly  in  need  of  AOE  dec  def  and  speed  lead/TM  boost.    So  Armina  and  High  Khatun.    Arena  team  should  be  High  Khatun  (lead  and  TM  boost),  Armina  (dec  def),  Kael  (nuke),  and  Ninja  (freeze  or  triple  hit  depending  on  enemies  left)  speed  tuned  in  that  order.

I agree with Qalam. In addition to that some tips to the things you said:

1. Stun set is a good choice for Bellower, if you use him in dungeons. You could take damage increasing sets (for example Critdamage, Savage) instead, if you make him your campaign farmer. But no frost set for Bellower. The frost set works completely different from the stun set: Stun works, when you hit, frost has a chance to freeze, if your champ is hit by an attack. Champs with provoke can use frost.
2. Frozen Banshee is a good candidate to become lvl 60 as well. She will help a lot vs. Clanboss with her poison sensitivity.
3. Be aware that you don't use fusion rares as food. You named Sorceress and Heiress as food champ. Indeed they are useless on their own, but they are part of the Relickeeper fusion. That epic is useless as well, to be honest, but fusing Relickeeper is part of the Arbiter missions. So, over some steps between them, Sorceress and Heiress are part of getting one of the best legendaries in the game. Sorceress and Heiress are not as critical, as you can get them back in campaign. But in general: be carefull while feeding your champs.

For  your  Arena  team  -  I  would  go

Leader:  High  Khatun  -  Speed  Aura  19

1st  move:  High  Katun  ---------->  TM  Fill  +  Inc.  Spd

2nd  move:  Old  Hermit  Jorge  -->  TM  Fill  +  Inc.  Atk

3rd  move:  Warmaiden  --------->  AOE  Def  Down

4th  move:  Ninja  ---------------->  AOE  Nuke

Definitely level up High khatun. I would also level up Tomb Lord. I think he is super underrated legendary. I use him everywhere. Arena and all my dungeon 20 teams and doom tower. He has aoe decrease def and att with his A3. And if you give him 100% crit he places 4 strong poisions on everyone with A2 and triple hit with decrease slow on his A1. Give him mastery that increases his  turn meter whenever his debuffs expire and watch the magic happen. Especially powerful on spider.

You mentioned considering Armiger, and commenting around whether he really is that good on Nether Spider.  From my experience, turn meter reduction is something you will want to avoid on Nether Spider, thanks to it's passive:

Elder Lurker [Passive] Counterattacks when enemy decreases Agreth Turn Meter. The damage inflicted is equal to the amount of Turn Meter lost. Will ignore 100% of DEF. 

This skill will punish you.  When combined with it's other passive:

Almighty Strength [Passive] Damage from skills that scale based on enemy MAX HP cannot esxceed 10% of the Boss’s MAX HP when attacking the Boss. 

It essentially means Armiger is not the best candidate for the role, and I wouldn't rank him up as a means of dealing with this.  He is a great champion for other content though, especially normal spider

save conqueror, for fw. You never know when rng in farming will stop u.....

You guys rock.   I'm starting with Frozen Banshee, to pair with Kael - poison sensitivity just makes him all the better, right?  Then you've given me a few cool choices for my arena - I'll look at where I'm at with books, etc.  Huge thanks.

By mid-game, you mean middle of early game.  FB would be a mistake to 6* next.  You have all attack champs, you need to start building support champs.

Armina would be great as the next 6 star. She has an AoE DecDef on her A2 and Turn Meter control, as well as Turn Meter control on her A3 with a chance to stun, and fills her own turn meter with that A3. Could help you in Arena, all the dungeons and most especially Spider.

My own Arena Team from your list of champs would be:

1) High Khatun, Speed Lead and Increase Turn Meter

2) Old Hermit Jorgg, Increase Turn Meter and Attack, available for revives if fight goes long

3) Armina, for her AoE Dec Defense and AoE Turn Meter control

4) Kael for that Nuke! Hopefully you've got him booked fairly well as a rare, so he should hit pretty hard by this point. You are looking for 4k+ attack, 100% crit and 200%+ crit damage as you're thinking of how to gear him to hit hard! With DecDef up on the enemy team, he should just end fight.

Good you ranked up Armiger, only 2 skills but they're both great. He is amazing in Frost Spider, and great in Ice Golem, when he is built with 100% crit rate and very high defense. He can hit fairly hard, and his Turn Meter control is excellent. His block revive is actually what I use to get through Frost Spider reviving herself.

So I guess for me my next targets for improvement would be Armina, then High Khatun (she gets another 7 speed for being 60) and then Jorrg and Tomb Lord, who like Arclite said is a leggo with a good kit.

Have fun! :)

Coffin  Smasher  will  be  useful  to  you  for  Clan  Boss.

Alure  is  a  Top  Tier  Epic  and  will  definitely  be  one  of  your  priorities  as  well.