Possible, but I don't think it is that simple, if it were, you'd think that Mother Winter would have retrieved her walking stick a long time ago.
I never got past "Harry Potter and the Blood Prince," for a number of reasons.. Anywho, Voldemort is also "He, who must not be named" So Jim may have meant merely that the Council do not want the name of the Warden to get out. If everyone knew who the Warden was, he/she'd be a target for someone who wanted to gain access. That is my theory, I don't know WOJ as to why he said that.
But the Council has never openly said, "if you go to the Island to become Warden we will kill you." On the contrary it was merely removed from the map, and it is the Island, "that must not be named" rather than it's Warden.
But is he really considered the new Dark Lord though? Or merely someone who in a very short time has acquired a lot of power that is mostly out of their jurisdiction, i.e. Winter Knight and Warden of the Island, oh yeah, starborn, and they cannot control him... The last is what scares them, they fear what they cannot control, if they cannot control him/her, ergo he or she will go warlock on
steroids... The starborn bit is interesting though, my theory is originally the Senior Council wanted a starborn created, but one of their own that they could control.. With Harry they were hoisted by their own petard.
I am not sure I follow about Mother Winter. If the staff can only be given, not taken for example - it's harder for her to get it back as she has to get someone to willingly hand it over. We don't really know how it works. But I suspect the other reason she doesn't get it is because no one would be stupid enough to walk up to her with it, and because she doesn't seem to leave her cottage now (or only very rarely at any rate) I suspect it's reasonably easy to avoid that sort of confrontation.
Jim might have meant they were guarding the name, but I will give you the quote and you'll see what he was getting at. I was wrong about what video, it's from the Barbarra Books Q&A
https://www.crowdcast.io/e/jim-butche and go to 24:37 and he starts to discuss who the previous Wardens were and how long, and then at 26:35 or so he outlines why no one has taken the position for a while. He didn't call them the next Voldemort in this interview, that was just my shorthand.
The point is though, that the Council didn't feel they needed to say they would kill whoever did it because a) they didn't think anyone would be crazy/suicidal enough to do it and b) because they thought anyone who would do it would be someone very, very bad. And then Harry stumbled in and made them all wonder again for the umpteenth time if he really is evil or if he's just an idiot.
I suspect Demonreach has another name (if not several). That's not uncommon, particularly for places and especially in this series. The Native American people would call it something different to Harry, the Council likely know an older name (perhaps Merlin's name for it) but there are many languages as well (not just human either).
I really don't know if Harry is considered the new Dark Lord, we will have to read the next book to really know that. But I suspect it was a part of the argument to kick him off the Council. As far as the Council are concerned, anyone who gathers a lot of power in a short amount of time out of their jurisdiction IS a threat, and therefore it's semantics whether they are really evil or not (to the Council anyway). They don't like opposition. Cowl hints at this being part of the reason the Black Council exists. The White Council, like any tyrant, is often largely responsible for creating their own worst enemy and arming them with the exact weapons required to bring them down. Tyrants can't help it really.
Whether they are actually concerned (although some surely are) that he will go Warlock, or that he is Black Council doesn't matter too much (with the exception that it might convince the more lenient allies of Harry like LtW and Ebenezar to side against him). They see him as a large, uncontrollable opposition with too much power and influence.
This gets to the heart of the issue with the White Council, and any governing body. The right of the individual versus the right of the majority. The White Council's authority (like any powerful governing body) derives from it's ability to enforce it's will onto others and still maintain control over their domain. They then must claim legitimacy either through divine right, or something similar, but often it's really just might. Why should the Council be allowed to kill Harry, or to control Wizards, or stop them doing time magic or whatever? Ultimately, the Council is a collection of individuals backed by a large(ish) and powerful group of people that have decided that it's better to have a governing body than let chaos ensue with no one in charge. Doesn't mean they are legitimate. But if most people have decided something, normally that's the way it goes. A single person's right don't necessarily count for more than everyone else's. But not necessarily less either. It's a huge, complex philosophical issue humans have been trying to solve since day one. We are more sophisticated in some ways today but it always essentially boils down to who has the power. And it likely always will, unless humanity develops into a far more sophisticated society beyond our differences stops trying to kill one another etc. I don't think we will get that far, but look how far we've come, so who knows?
I think you're probably right about the star born thing. Harry didn't work out the way they hoped...but then again he isn't a complete failure either. The Merlin is famously cunning, and Jim has said he often is far less direct than Harry suspects. A career manipulator and politician. Harry might be serving the Merlin's aims despite not actually intending to. The implication is that he meant Harry to destroy the Red Court, despite appearing to try and hamper Harry.
It just occurred to me that both Maggie and Ebenezar before her could both have been Warden's at one time or another. Other candidates are Simon and Justin, and potentially Rashid and someone (again, I would think Maggie). Maggie and Justin just seem like the most likely to have preceded Harry given what they both were like and got up to.