Coldheart

i still use apothecary, coffin smasher, and bellowed in fw 21 for the 'using a rare' advanced quest.

Bellower is my 7 sec brutal campaign farmer

I see reliquary, gravetouched, heiress on cb teams

If by "end game viable" you mean those that you will use on the regular, as you reach endgame, the list is short:

1. Coldheart.

End of list

If you mean to include everyone you can find use for in the endgame, then it's Apo, Reliquary, Frozen Banshee, Bellower, Warmaiden, Heiress, Elhain, Soulbound, Master Butcher and Fellhound. The rest are either straight up food or can be farmed if needed.

Reliquary Tender - I use in hard mode doom tower for nether spider

Coldheart - lots of places, fireknight, spider

Frozen Banshee - clan boss

Coffin Smasher - clan boss

Bellower - campaign farming AOE

Apothecary - I sometimes use in end game teams if I need speed and big single target heals. 

I would use rest for food.   

Frozen Banshee is  on my unkillable CB team so I use her everyday :/ if I want to 1 key

Soulbond Bowyer ... she's the queen of some secret rooms at Doom Tower hard

This list right here has everything to save.

Renegade is top tier for late game dungeon speed runs, unless you have a kymar.

Painkeeper is also used in some clan boss setups.

Both really need specific comps and outside these they are not worth building unless you intend to run these comps. I would keep a copy  of each since vault space is large now. 

Coldhearth , must keep 

Everything else follow above advice and feed the rest imo (esp if it helps you get more lv60)

Thank you ALL.  Excellent advise and direction for next steps. I appreciate it. Onto the food factory. lol

Good luck with it! :)

I have 2 coldhearts built that i use regularly, heiress is in my mythfu team for clan boss and apoth may still be in my FW team

Hope you didnt eat reliquary? i rate her for mid game at least.

How can you tell when you are in Endgame?

End game is open to interpretation

For me I felt I had got to end game when I had finished all DT hard rotations and secret rooms. Others might feel end game is finishing fw, or getting ramantu. It's just a term and I wouldn't worry about 

I feel coldheart is good but still squishy, pretty easy to knock over by herself.  I have seen big power, and several of them together in the arena and they still go down with hardly much effort.  With me i have to pair her with scyl to keep her upright, or anything else that can help with a shield.  Spiders eat her like crumbs.

JMHO

She's not meant for arena. Coldheart is used in Spider 20 / Spider 25 teams that clear in ~10 seconds. You're not getting hit, so you won't die.

This goes for just about everyone, btw. You don't build damage champs to survive. You build them to just do as much damage as possible. Other people are there to ensure they don't get hit - either because you're CCing the other team, or because they aren't even getting a turn before they die.