I have heard this teaching, but I have noticed some questions drifting here and there in my head about it.

One aspect of japa seems to be to develop concentration. Concentration itself is such a miraculously powerful thing. A being who has reached a masterful realization of concentration without the use of japa manifests certain states of being and accomplishments which seem identical to one who has realized the same through japa?

I have seen it said different ways, but essentially the same meaning, that "All the divine potencies are hidden in the Lord's name". Now is this just a devotional way of saying, that with the practice of perfected concentration one becomes aware of a state of being/existence that is "god". In the case of the use of japa it then appears that japa is invested with power. Whereas what has actually happened is that the japa has affected such a profound change on the mind that one experiences a "god" state of being and then says, wow that mantra is powerful.

Or, is there actually a sort of invisible audio frequency like object that we call mantra, basically a sound vibration, that somehow is itself or carries within it some sort of further mystical magic or power? That other sounds and noises just don't have that power, but only certain sound vibrations have the power. And among those that do have the power, some have the power greater than others (to reach full enlightenment, vaikuntha, etc.), while still yet others that do have a certain kind of power don't have the kind of power to bring enlightenment, etc. only mundane accomplishments.
Now in Swami Sivananda's text it says "hidden in the lords name". This does make it sound as if there is a medium - sound vibration - which like a vehicle can carry certain subtler energies? But words, language, can be misleading, misunderstood, especially with subject matter like this.

Put a different way, the question seems to be: Is the potency/power found in the mind which is perfectly concentrated? Or is the potency/power found in some apparently external subtle energy carried like a passenger in sound vibration? Or both? Are there two things happening here or one? Can one exist without the other? Is there power in the mantra without concentration? Is concentration alone enough, equal to mantra?

To put it even more simply. Is it better to practice with japa as a focus or something like breath? Does mantra have a power not found in the breath? I have heard that breath itself is a subtle mantra. If that being the case then how are certain mantras more powerful or of different quality than others? How about objects of focus like yantra, surya, etc. Is a mantra hidden there as well? If not is the power derived from non-mantra focal objects exclusively found in the minds concentration?

Further. If there are certain mystical potencies of various qualities being invested in certain sounds and things, how is this happening? Is there some great invisible ego/personality type being sitting behind the scenes making arrangements and designing things in a specific way? If so is the design totally arbitrary and artistic, based on creative personal decision. Or is there a certain framework that such a being is required to work within? And if that being the case, is the framework itself somehow more powerful than that being doing the creation or is it a framework that the being itself has chosen to limit itself within? And if so can it arbitrarily break the rules?
Actually I know these are basic fundamental questions that must accompany any god concept.
I guess as long as my mind works in terms of concepts the questions will remain.
I suspect the only way to really clarify all such questions is to realize freedom within concepts through enlightenment / practice itself.

But, it would be greatly helpful to be informed, from a truly realized being, about the reality status of mantra vs. breath. The basics of how they function and which is really best. Considering this is something, if one were really serious about it, would account for a majority of the time one uses in this entire lifetime. It is quite a decision to make. One tends to think that once a path is followed for say 10 years, one should continue with that path/practice and not change? Or can one practice breath for 10 years and switch to mantra with no difficulty? Or can one practice each day, half breath, half mantra?

The final thing I would say. I have little previous conviction about any of the above matters. I really know very little directly. I do know that conviction itself is useless. So in regards to sharing information or presenting arguments with the purpose of affecting or changing anothers convictions. It is a useless endevour. All we can do is share our own direct experiences or pass on information that we feel is genuine from another who's experiences are real.