I agree with Rachel's observation that Flammifer, in their first post, conflates two different instances of Gandalf visiting the Golden Hall. We have the first instance that was discussed in the podcast episode where we only have described that the king 1) would not listen to Gandalf's warnings and 2) his final decree that Gandalf take a horse and be gone, The second instance of course we will not get to for another few years being in the middle of book 3, where Gandalf the white and the 3 hunters finally reach the Golden Hall.
However to take Flammifer's observations at face value (and to get a little ahead of ourselves) I would disagree that Gandalf crosses the line here of "dominat(ing)by force and fear. I feel this interpretation is both A) takes too strict an interpretation of dominating by fear and force, and B) is actually not true when reading the following paragraphs and seeing his magical effects in context. I will deal with these one at a time.
A) I believe that 'dominating by fear and force" is too high a claim for 1) darkening the sky to prove a point and 2) knocking out the king's advisor (I'm not sure he actually does but we will deal with this later). this is a singular instance and I feel this is more in line with Gandalf's help to Bilbo in book1, ch1. In fact, the darkening of the room in Bag End is quite similar to the darkening of the windows of the Golden Hall. I believe theatricality to emphasize a point is well within the Istari's purview to use, and I think that claiming Gandalf is crossing a line here places an extreme constraint on their ability to serve the people of middle earth. I doubt Tolkien meant for the wizards to be so passive. (i will also note here that in line with other observations from Corey Olsen, the appendices were largely written to explain the text, and not the other way around.) as we can see from Saruman's example, his plans of Dominating by fear and force involved a months-long campaign of subterfuge by his ally?/subordinate? Grima and potentially later forceful coercion by his own army of Uruk-Hai and Men. Gandalf himself leads armies in the siege of Minas Tirith, but does so consensually by the armies of Gondor, and does not presume to continue directing them once rightful leadership is restored to Minas Tirith. I would even go so far as to say that I would not place the actions of Jackson's Film Gandalf, knocking out a Hysterical Denathor, and taking over the leadership of the defence of Minas Tirith beyond the bounds of the stated restrictions from Appendix B. That action (clearly not in the books) is of course far more direct in taking over the armies of Men, but I think would fall into acceptable use of force given the circumstances.
B) however returning to the incident in the Golden Hall, I don't think he is dominating by fear, certainly not Theoden, and probably not Grima. Gandalf darkens the room, sunlight is blotted from the windows and the fire fades, while Gandalf shines in the room, then when Wormtoungue hisses about Gandalf's staff: "There was a flash as if lightning had cloven the roof. Then all was silent. Wormtoungue sprawled on his face." It is unclear to me what exactly happens to Grima here, did Gandalf literally knock him out with magic that looked like lightning, did he actually make lightning and Grima faints or cowers on the ground (like Bilbo thinking about dragons in the hobbit)? I'm not quite clear on that. however what I am quite clear on is that following this demonstration Gandalf asks Theoden to listen to him, releases his darkening spell in one window to show the light from the sky and tells Theoden that all is not dark, and he has counsels to give him if he will listen and not despair. In fact, he does not give him command or council until Theoden himself walks out of his Hall and into the daylight, where he counsels him on breathing the free air, and to stand tall, and only when Theoden asks "what is to be done?" does Gandalf give any advice or counsel on matters of State and War. In this I see Gandalf exercising quite a lot of restraint in dealing with Theoden, after finally shocking him to his senses Gandalf uses no coercion or force to make Theoden do anything, and gives the king his space to make his own decision. As for Grima, well, Gandalf is definitely using either force or fear to silence him (in some capacity) but one man does not constitute "dominating Elves and Men" (at least in the sense of races or nations). And even later once Gandalf has accused him of being Saruman's puppet, even he advises to let Grima be given a horse to go where he will. I think we can see here Gandalf's tendency to nurture and help people make their own decisions (like Bilbo giving up the ring, and Frodo choosing to take the ring in the council) even if momentary force is needed to loosen the claws of shadow that grips those he helps and counsels.