I have only read the two Allies of Humanity books, and flipped through his earlier works. What you quoted above can also be found in the first Allies book:
“This message is coming through one man. Why aren’t you contacting everybody if this is so important?"
It is merely a matter of efficiency. We do not control who is selected to receive us.
That is a matter for the Unseen Ones, those whom you could rightly call “Angels." We think of them in this way. They have selected this person, a person who has no position in the world, who is not recognized in the world, an individual who has been chosen because of his qualities and because of his heritage in the Greater Community. We are glad to have one through whom we can speak. If we spoke through more, they perhaps would disagree with one another, and the message would become confused and lost.
We understand, from our own studenthood, that the transmission of spiritual
wisdom is generally given through one, with the support of others. This individual must bear the weight and the burden and the risk of being so chosen. We respect him for doing this, and we understand what a burden it can be. This will be misconstrued, perhaps, and that is why the Wise must remain hidden. We must remain hidden. He must remain hidden. In this way, the message can be given, and the messenger can be preserved. For there will be hostility towards this message. The visitors will oppose it and are opposing it already. Their opposition can be significant but will primarily be aimed at the messenger himself. It is for this reason that the messenger must be protected.
The risk is between corruption of a single message being interpreted differently by different messengers, and accusations of guruism if the message is kept pure by dissemination through a single messenger.
If anyone has a pipeline to anything, they better be tapped into something pure and be careful not to corrupt what comes through. What the case is may be determined by the quality of the material. We know of examples where some delusional egomaniac claims to be the only source of salvation. Fortunately the material they generate is baloney. The question is whether there could ever be a pure message delivered through a single honest messenger.
If we're talking about a fourth density STO group of alien observers, which is what the Allies basically show themselves to be, then it makes sense they would be given a single appropriate receiver to disseminate their package of knowledge. Even in offices, a single message is given to a single messenger to deliver to avoid confusing the recipient. To say that Marshall happens to be the receiver for that group does not deny other STO groups sending their knowledge packets to other receivers, nor does it deny the ability of people to tap into a more universal higher source to essentially derive the same concepts, sources like their higher selves or even the "Unseen Ones" which in this case are sixth density STO. I think it depends on the limitations of the source and the specificity of the message.
Regarding the New Message, the claim is that this particular message from God was sent only through Marshall. Does that mean no one else can receive communications from God? I doubt that's what it implies. Personally, I question the claim and don't really pay attention to that part of the material. If the rest has good, accurate, timely insights then okay, that part I appreciate. In my view, the personalities should always be secondary to the material content.
Yeah, there is always risk of ego corruption and source hoarding as time goes on. The Cassiopaean material is a good example of that. It's tough when a person is not only responsible for putting out info, but also be a leader to a group of seekers interested in the material. The seekers may put personality ahead of knowledge and turn the leader into a cult figure. Some leaders take the temptation and play guru, others get distraught at this and give up to sink back into anonymity, and others are somehow wise and strong enough to be good and wise leaders.
If someone claims they are the only source of something, then that will be proven if they are the only ones putting out those ideas. If so, most of the time the ideas are unique because they are subjective garbage no one else cared to make up, but occasionally they are unique because they tap into a new body of unrealized truths.
Acquiring fringe knowledge is like digging for diamonds in a mine field.