Questions About Specific Sims in the Manage Households Tab

@AalyahReed 

In short, what you want to do works.  I have done exactly what you want to do with every one of the sims you've mentioned and even a few other NPCs that I want to keep in my game in the same roles forever.  And I've even done a little more.  I made those that were not already immortal, immortal which is most of them and made them not age as well.  And I have edited most of them in CAS.

I keep all of them under the My Household tab in Manage Households as well as a few other NPC like the OR hermit, butlers and university professors.  As long as they are marked with the light green plumb bob icon in MH, they remain NPCs and keep their NPC roles.  You can edit them in CAS with no problem.  With few exceptions, if you ever move them into a house and enter the household in live mode, they will lose their NPC roles though and the game will make new sims for those roles.  And I found that if you merge some of them into a household with others, they will also lose their NPCs roles.  So just let them be as single sim households (or in the case of the HonB Mayor and her son, a 2 sim household) and never play them in live mode.  You can move them into a house and visit them, just don't play the household ever.

Keeping them under the My Household tab does keep them from being culled and used for some other roles.  They may still get grabbed for things like walkbys, jogging and gym patrons but otherwise they retain their NPC roles.  There is one thing though, if they are not immortal out of the box, they will still age and die and then new sims get generate to fill the NPCs roles.  They are not all the same though.  Father Winter is immortal from the start.  Butlers will not age but can still die by accidents.  Most of the others will age and die.  There is a way to stop that though if you use mods, specifically if you use MCCC.  You can give them flags to stop aging and make them immortal.  Which is what I have done for mine.  It can be done and it works to preserve those sims in your game forever.  And you can use the MCCC options on the fly at any time you see the sim out in the world.  

So, in the order you mentioned them, here's what I did.

HonB:  Kim, Mayor, Mayor's son, Michael Bell:  I gave them stop aging and immortal flags.  You don't have to do anything with Agatha or Agnes.  Those 2 are hidden and immortal.  You can't edit them in CAS.  For the Scotts, I did not realize they had NPC roles and because I always go into every household we get in new pack to make all the premades playable before I actually play in a new world to keep them from aging until I get around to them, I inadvertently caused them to lose their NPC roles as the bar owner by making them played and my game made new ones.  Didn't even realize what I'd done until quite a while later.  I liked the new couple owning the bar so I just let it be. The Scotts are just another family now. 

 RoM Sages:  Morgyn, Simeon, L. Fabia:  I gave them stop aging and immortal flags.  I made the mistake of combining them into a single household in one of my test games, and they never showed up at Magic AHQ again.  Thankfully, it was in a test game, not my main long term save.  Just leave them as single sim households.  I also have 3 other sages as well that the game made for a total of 6 where I did the same thing.  If you stay in Magic AHQ for a long time when you visit, you'll notice that they sort of work in shifts and after a certain amount of time, they will leave and be replaced by other Sages.  Stay there long enough and they will return.  It really is like a shift change at a factory.  One thing I found though, is giving the 2nd shift Sages a makeover in CAS doesn't do any good as they will always revert to however the game dressed them whenever they show up in Magic HQ.

IL Mermaids:  In this case, they are just like any other sim and they age and die.  They do not have NPC roles and never did.  There is nothing special about them.  They're just mermaids and they died one by one and I lost them all.  I was able to get them back by downloading them all from the Library and combining the households into one household of just mermaids.  Then my game grabbed them to be stall vendors since they didn't have careers.  And because they were stall vendors, they never showed up out in the world in Sulani as mermaids, only as stall vendors.  So I moved them into a house and gave them a regular career, moved them back to not in world, and made them unplayed.  Now they show up as mermaids doing mermaid things.  I did give them stop aging and immortal flags so I will always have them.

DU Professors:  I gave them stop aging and immortal flags.  I have 2 of them.  I gave them full makeovers and unlike the sages, they are wearing what I gave them in CAS whenever I see them. 

Father Winter:  Clement Frost.  FW is immortal from the start but it is possible to get him into a household via asking to move in, marry, etc. and the game will make a new FW and Clement becomes just like any other sim and loses the NPC role.  In my case,  for a while, I had made all the immortal sims mortal via an MCCC option so he died eventually.  The game made a new FW.  Several in fact.  But unlike Clement, the new ones are hidden and you can't edit them in CAS normally.  Eventually, I wanted Clement back.  So I tried something.  I created a new game and it gave me a new Clement in the Other tab of MH.  I took him to CAS and saved him to my library.  Then transferred him back to my main save.  Put him in a house, then immediately moved him out back into not in world without ever entering the household in live mode.  Crossed my fingers and the next Winterfest, he showed up as FW at the house I was playing.  So that can work, at least for him.

OR Hermit:  I just liked one of my hermits a lot and wanted her to be the one that always showed up.  But my game just created a new one every time I went to the Deep Woods even though the others were still there under Other in MH.  So, I tried something.  I made her immortal and deleted the others.  Since then, she's the only hermit I have and shows up in the Deep Woods every time now.  Exactly what I wanted.

Butlers:  All butlers have aging off when they are made by the game.  They can still die by accident though.  Mine keep dying by electrocution from trying to fix broken robovacs.  There is a current bug with them so they can't be repaired but the butlers will keep trying to repair them until it kills them.  After losing the third one that way, I made them immortal.  I also keep an eye on the robovacs and replace them or use the debug cheat to fix them when they break.  Other electronic items do the same thing too so you have to keep an eye out.   

Neighborhood Stories:  I have the stories off for the sims under the My Household tab of MH on for the Other tab.  That way, none of the sim in My Households, played or unplayed, are affected by the stories. 

Hope this helps.