I guess the theory is possible, but I personally don't see much support for some the ideas, and some of the reasoning I find either disagreeable or inconsistent. But I like the unique suggestions brought up by this theory, and I think the theory's creative perspective is a positive thing even though many of the ideas may not be found acceptable by the majority.
Already though, other posts have addressed some of the disagreeable points, such as why would the Dark Soldier's heart be placed in a four year old boy and such.
I guess my main question is that if the Dark Soldier's heart was considered dangerous, then why not just dispose of it... or seal it away elsewhere. I must admit, I'm not a fan of the popular concept of "dangerous objects being sealed within individuals to serve as appropriate hiding places". It's a possible idea, but it's a bit over-used in stories, and sometimes doesn't seem justified by adequate reasoning, since other options may still be available.
Again though, you bring up some unique ideas.
Horizon's Knight said:
In the KMIIFM+ secret ending, Ven knocks DS' helmet off. Ven was shcoked by who it was, so it would be someone we least expect.
That was a TGS or Jump Festa Trailer from a year or two ago. Besides that though, Ven seems like he was surprised by the face behind the mask. That doesn't mean though that it's a person "we" would least suspect. All this may indicate is that it was an individual who "Ven" wouldn't suspect.
I think I missed this though while reading the theory... why is Ven surprised by the face behind the mask? How do you believe the Dark Soldier's face looks like, and why do you think Ven would be surprised by it?