As an outside observer, without a "horse" in the race so-to-speak, I've found myself interested in the history of Daito-Ryu and Aikido as the availability of information on my own art is thin at best.
Having read nearly everything Stanley has put out on the topics and much of what Peter Goldsbury has written, it seems to me that Ueshiba's Aikido died with him and is likely never to be fully "reconstructed" as Mark Murray nicely alluded to.
The development of Ueshiba's martial skills was a result of very unique circumstances and instruction. Ignoring the spiritual aspect entirely, simply the fact that he received Daito-Ryu instruction directly from Takeda resulted in exposure to a non-standardized, and changing curriculum. The Daito-Ryu Takeda exposed Ueshiba to was not necessarily the Daito-Ryu transmitted to Tokimune, Horikawa, etc..
Add to the fact that Ueshiba brought in elements from other arts and you have a martial foundation that cannot be replicated by a modern practitioner. I would argue that even if Ueshiba hadn't found himself drawn so deeply into Omoto-Kyo and focused all his energy and time on transmitting his interpretation of DR or early Aikido he would have had a difficult time truly passing on his particular skill-set.
How would one go about trying to developing it today? You can turn to Daito-Ryu to try and fill in the gaps, but that in itself opens a can of worms. Which branch of Daito-Ryu would provide you with the appropriate skill-set? Which interpretation is the genuine article? Didn't Kondo claim that he was the only one who was shown true Daito-Ryu from Tokimune?
It seems like interest in bringing outside elements into modern Aikido in an attempt to reconstruct Ueshiba's skills has exploded. Nearly every thread seems to include some mention of internal power/aiki skills of some sort. Veteran Shihan like Ikeda are consciously pushing to evolve their skill sets. A friend recently returned from the summer camp in D.C. and said that Bill Gleason's Aikido was spectacularly different than just a few years earlier.
In an attempt to restore Ueshiba's Aikido, modern Aikido seems to be consistently moving towards an evolution beyond it's origins. Even if Ueshiba's skills can't be reconstructed, it doesn't mean they can't be surpassed.
Bill Is changing, Ikeda and Saotome have not only commented but have expressed an interest in the source of the change.
A few comments about a few points you made.
While I agree that no one knows what Ueshiba was exposed to compared to other Takeda students. We do know that Sawaga was considered his equal, some who felt both considered Sagawa better Kodo was a giant and Hisa and Tokimune were both considered quite good.
As for what else Ueshiba did that may have added or arguably detracted from Takeda's aiki, no one knows. It's more speculation. Once you have the the body method, you will grow on your own with further research. Each of Takeda's five greats talked of their own continued growth. It's the way it is.
We do know that when Ueshiba decided -to teach- he decided to teach DR throughout his career and handed out copies of the Hiden Mokuroku scrolls to all of the prewar students. All of those students are mistakenly called prewar aikido deshi, when in fact they were Daito ryu students (again as the artists behind Budo renshu stated "We all thought we were students of Takeda Sokaku under Ueshiba sensei"). After his retirement as stated by him, his techniques changed, I suspect it was for that reason that he lost Mochizuki, Shioda, and Shirata, but that has never been made clear either. Even though his more open and large circular movements - with a more flowing protective quality- was a dynamic shift from DR waza, but his aiki was unmistakably still Daito ryu.
Your last point about surpassing Ueshiba should be everyone's goal. Power is not all the same, even though some like to say it is all the same, nothing could be further from the truth. The Chinese just like the Japanese, argue over methods and uses and purity, yadda yadda. And the pressure Ueshiba faced is different than the pressure a modern adept faces. I am confident that were Takeda and Ueshiba alive today, they would have followed Sagawa into exploring the body method into wrestling, boxing and other more modern methods.