I believe that what really separates an internal art from an external art is the *teaching paradigm* itself. I think Aikido is often misunderstood as an internal art due to its complicated nature and emphasis on softness/relaxation.
The comments aren't bad, Chris, but you're arbitrarily assigning nomenclature in a tricky arena. The so-called "internal arts" are the arts of Taiji, Xingyi, Bagua, and a few others. About the only thing that defines them is that they use certain approaches to ki/kokyu skills and other arts use different approaches. In terms of "internal strength skills", the "neijin", all the Asian martial arts have them. It's de rigeur. And all the tenets of all the Asian arts make it clear that they're in the know (that includes O-Sensei's douka, koryu secret documents, etc., as far as I've ever seen). Can we do most Asian arts using "normal strength" to execute most techniques? Yes... and that is the wrong way to do it.
So should Aikido use "neijin", "internal strength"? Yes. Is it one of the arts that fall into the direct category of the "internal arts"? No. What's happening is that the terms "internal" and "external" are loosely used to define two different parameters and your discussion appears to confuse the two usages.
Until you straighten that part out, the discussion can't intelligently progress.