Hi Kirk. I found your theory very interesting and wanted to add to some key points in your arguments, and as such have sequenced them rather out of order for ease of reading.
I find it suspicious how Shanks was implicated into the whole Ace and Teach fight. We know that Rayleigh knows about the void century, the will of D, he knows everything. It is the same for Shanks, problably, since he was in Roger's crew as well.
Going with this, I believe that Shanks try to find "that man" that Crocus, and Whitebeard were talking about. Of course we know it's Luffy, but Shanks as a character, I think his duty is to find that man, and he kinda did it with Luffy and the whole bet about the new generation.
I think you hit the hammer on the head of Shanks being the first selected person to carry on Roger's will, with 'finding that man', the man who will reach Raftel and find One Piece.
During the time that he met Luffy as a kid, he was already sporting the scar, which meant that the confrontation with Blackbeard had happened since. Going by what you said; I agree with your theory that it was his duty to find 'the man' that Roger was waiting for. Who is that man though? Well, it's obviously Luffy at this point, but what attributes or qualities did Shanks see in common with the two that caused them both to be of such interest to him?
I believe that only the 'ability to hear all things' will be the necessary key to **finding/b] Raftel, but we know that the story doesn't stop there. Luffy, upon learning the True History and likely activating the weapons, will make his move along with all his allies against the WG; something Roger never managed to do. However, judging from your theory of Shank's true duty, he likely also needs a D as the Will of D has been both mysterious and instrumental throughout OP. So he'd be looking for someone who can 'hear all things' and bears the initial of D. I'd like to take the time to add my thoughts to the story below.
I think that longtime ago, after Roger's death, Shanks met Teach and have seen in him the man that was awaited by Roger and co. Teach is a D, and Shanks could have been interested by him just like he was with Luffy. Shanks made a mistake and picked the wrong guy, that's my guess. Why is Teach so confident about his fate? What does he know about the D? I think he knows almost everthing, thanx to Shanks. And that's why Shanks might be so concerned by Teach. He was still young, and he made a mistake. He told him everything. Learning from his mistakes, he then did the opposite with Luffy, telling him nothing but just giving him hope and motives.
Blackbeard seems to trust fate a little too much, especially in very dire situations, and that kind of behavior is reflective upon someone who likely already knows what is to happen; that is to say, your theory of him being a failed candidate of Shank's knowledge, trusting in his destiny far too much.
We still don't know a lot about Blackbeard. What if he also, like Luffy, has the ability to hear all things? What if Shanks recognized the initial D (which wasn't unheard of) but also saw the much more rarer ability in Blackbeard and acted hastily? Per Roger's instructions and criteria, Blackbeard meets them; however, his personality is such that he is hell-bent and obsessed on world domination with this knowledge (a stark contrast to Luffy's ambition) after obtaining it. After all, telling the wrong kind of person that they're "The One" figuratively, as you said, would make him corrupted.
Continuing your theorycraft of what Shanks may have been thinking: He meets Blackbeard, recognizes his potential and decides that to fulfill Roger's will, this man needs information to do it and do it possibly fast. However, things don't work out yet Shanks won't stop Blackbeard even though he could likely wipe the floor with him (I'll touch back on this later). Years later, he meets Luffy, who also happens to be a D.
The comparison between Blackbeard and Luffy is quite common, and from the get-go they are clear contrasts down to the wire. This also meets true by your reasoning; Blackbeard is the one with the knowledge, the template and the will (ability to hear all things, Will of D, knowledge of Void History and weapons), Luffy is instead simply told to 'become a great pirate' and filled with promises and hopes instead. Shanks didn't quite see the potential until Luffy says something that sparks Shanks interest.
Both Rayleigh and Shanks, survivors of Roger's crew are aware about 'the man that Roger was looking for', which is why this conversation is of such interest. Shanks would not be saying such words on a whim, he'd be seeking out Rayleigh to tell him something big, something potentially world changing.
Perhaps he was underhandedly implying to Rayleigh that Luffy was possibly the man that Roger was looking for, which is why Rayleigh was also super keen on meeting Luffy.**