Bob heavily implies that it is the MANTLE that is immortal, not the wearer. So the mantle reforms, and the wearer with it normally. On Halloween (and in other highly specific circumstances) the mantle doesn't protect the wearer, so they die and the mantle can change. But because it is immortal, that is why it ends up shaping the wearer. I suspect that if the wearer were to reject the mantle, then the mantle might not even protect them forever. Molly will eventually end up much the same as Maeve, as that is what the mantle tries to do. Unless Molly can change the mantle or rid herself of the mantle, she won't BE Molly forever. That much has been said in different ways throughout the series.
Think of it like the office of the President. The President, as an office, is an idea. That is not easily destroyed. The occupant, however, is very much vulnerable. It is not the occupant who has power, but the position. But that power is next to useless unless there is an occupant to direct it. We always respect the position (as a rule), but often not the occupant (depending on our various leanings and values). In psychology, this is called authority or social power (Positional Power). It's seen as legitimate by society (generally, although not always as sometimes Monachs and Leaders are de-powered by society for various reasons, often because there is too much concentration in a single role).
Everyone has a degree of Personal Power, which is that which you have in you. It comes from the physical strength of your body and from your personality, your charisma etc. Sometimes this is split between two ideas: Referent Power and Personal Power, with Referent Power referring to interpersonal skills and ability and Personal mostly referring to the natural physical abilities.
Then there is Expert Power, that which is granted by knowledge and experience and skill in a field or fields. We respect the great masters of sport, art and academia etc because we can see not only how much natural talent they may have, but also how much they have learned about their area and how much skill they have built up. We trust and respect and often admire these people because they are the gatekeepers and the pioneers, those at the pinnacle.
Reward Power is another type, that mostly depends on the ability of the individual to give or withhold valued (generally material) rewards. Sometimes this is split between Reward Power and Coercive Power, with Reward merely referring to the ability to give and Coercive referring to the ability to withhold or even disadvantage the receiver.
In any case, immortals in series are not the same beings as the rest. They mostly cannot be killed and will not die of old age. Whilst there are likely many supernatural beings that are extremely long-lived, and perhaps many cannot die of old age or disease, they can be killed with force. Immortals are a different category. You can atomize them and cast their dust upon the universe but they always reform. This is heavily implied to be because of mantles, but not necessarily. We don't yet know how Angels or Fallen or others fit into this.
When the supernatural in the series refers to Mortals, they are referring to beings with Free Will. They don't mean dogs and cats, generally speaking they mostly mean human beings. There is a good argument to be said that the Forest and Mountain People (like River Shoulders and the Genoskwa) are actually in this category, not in the least because they can pick up a coin or perhaps a sword. I would say all scions and Changellings (pre-change) and beings like Thomas probably are in that category (although not his demon).
Whilst Vampires and most Faeries are not immortals, they do appear to not die of old age or regular mortal problems. It clearly takes either crazy amounts of power and/or a really long time to become immortal. A curious case are Fae that are "Eldest". The title passes on to the next oldest when the previous one dies, but the being (and it's power) seems to pass away. My guess is that it is the barest sort of mantle, but not really powerful enough to be significant. But with added power, and perhaps time, would become a real deal. The difference with a Queen, let's say, is that they're mantle makes the office-holder immortal too. The only exception we have seen is the Knights, and I suspect that is because it is built especially to NOT make the holder Immortal, because that would make the position useless for it's purposes.
The Knight IS the Queens (all 3, and perhaps all 6) ability to really affect the mortal world in a meaningful way (i.e. Choice).
Molly is no longer really going to be worried about Black Magic taint, as she isn't really human anymore. She could possibly Choose to not be a Queen, but I suspect not, which means she is Choosing to sacrifice her Free Will. Not all at once, but over time.
