I would not equate "inner strength" with "internal strength". Those are two completely different things to me. And "internal strength" doesn't mean being the strongest person alive.
If you'll step back a moment with me, I would look towards Ueshiba's martial skills and what he accomplished. He had people push on him and try to topple him over or to make him move but none truly succeeded. That is one qualifier of defining "internal strength". It has to do with training the body a specific way to achieve that capability.
"Inner strength", on the other hand, I would look to Ueshiba's spiritual pursuits and see how he manifested that. For instance, he took the principle of Daito ryu to break and kill a person and he added another choice of sending the person outwards unharmed. That takes an inner strength to make that choice (understand that there has to be a martial capability, but that is another matter), to be the better person, to change how people view budo.
While I think that Kisshomaru altered his father's spiritual ideology to allow for a worldwide appeal, there is still that "inner strength" message in Modern Aikido.
What is lacking in Modern Aikido is the martial capabilities that Ueshiba had and that is where "internal strength" comes into play. "Internal strength" is a core body skill that allows better martial training in whatever art you choose to do. It only very tangentially touches upon the spiritual/mental/emotional concept of "inner strength" as learned in Modern Aikido. The two are very, very different things but are also very, very complementary. One does not invalidate the other.
As you noted, "inner strength" can be found in aikido training all over. It is part of the mold. "Internal strength" is not found in aikido training and can only be seen in a very few Aikido Greats such as Ueshiba, Shioda, Shirata, etc.
The very apt questions about whether someone has good "internal strength" or not are these:
Can you have a sumo champion try to push you over and you not only don't move, but you pin the sumo champion such that he admits defeat? (Ueshiba, Takeda)
Can you have 5th dan judo champions try to throw you and you not only don't get thrown, but you toss them at will? (Takeda, Mifune)
If you don't have a clue as to how that is at all possible, then you are not training "internal strength" and no amount of "inner strength" training will get you there.
That isn't to say "inner strength" is worthless. It is not that at all. I'm just trying to show the difference between the two and why people are going out and training "internal strength". They have resources for "inner strength" training in aikido, but not for "internal strength".
It isn't a matter of being the "strongest person alive" but a matter of trying to follow Ueshiba's vision. He not only had the "inner strength" but he also had the "internal strength". They were both seamlessly intertwined.
Why are you searching for internal strength? Why is it missing from your Aikido training? Why are you going outside your art to find it? When I read the threads about this training it makes me wonder what you are lacking that you have to go find it.
Once when I was joined in the conversation I was told I don't have it because one of my students doesn't have it. If you asked me about that student I would agree. His heart was not in it. Aikido training lets your inner strength out. You don't have to go to an expert to find it.
Aikido training for me is a whole practice…the development of correct feeling is just part of the process in becoming what Aikido is molding me into. Are you missing the point in your impatience to be the strongest person alive?