PCE: How can anyone reject Him?!?

[childeye 2]

All the rest of the 'content' of salvation and sanctification. Though the story has some important aspects, it is not a full theological statement, eh?

Hmmm. I grant you that it's only a circumspective nutshell of Eternal matters being described using temporal terms. Theistically, it is consistent with the faith that God is good and trustworthy.

Do you see any connection between the son gone astray into Sin City and the sheep goes astray into sin?

I do, in that I define sin as any direction in separation from God. They both thought the grass looked greener on the other side of the hill, so to speak.

Do the two metaphors fill in our understanding? Does the wayward son being in the Family before leaving for Sin City instruct us about the sheep being in His flock before going astray into sin, not merely by returning* to the Bishop of his soul???

As I said it's a nutshell representation, but the premise is the same. The son wandered his own way and so did the sheep.

* 1 Peter 2:25 - For ye were as sheep going astray: but are now returned unto the Shepherd and Bishop of your souls. Again, to return one must have been there before, at least, according to the normal use of the word. Therefore, in this verse, it would be normal to infer that the sheep that had gone astray, were, at one time part of the Shepherd's flock but had strayed away from HIS care. Since I am sure that the Shepherd was not negligent, the straying away from HIS care must involve some rebellion.
Were they forced to go astray? Doesn’t seem likely, does it. Was the Father or the Shepherd lackadaisical in their duty, that is, was their fall into sin His fault, His lack of attention? Not at all. So some rebellion must have been involved in the son and the sheep ending in sinfulness…

Okay, so it does not go into enough detail of how mankind goes astray. As I see it, distrust/unfaith and pride would precede rebellion. Rebellion certainly does not proceed from God's counsel, just as you say. But I think what we're actually referring to is ignorance and vanity, which can begin in the gradual slow motion nuance of taking God's providence for granted.

I posted this earlier:
Romans 8:20
For the creature was made subject to vanity, not willingly, but by reason of him who hath subjected the same in hope,

IF they are conceived/born into sin, that is, NOT by making a sinful choice of their own, when can we understand how they became sinful by their own rebellious choice, not by any fault of the Father or the Shepherd?

I believe there exists a scenario between the Creator and the creature that is mutually unfair, but it's no ones fault because it's a matter of circumstance. For example, for the creature it's possible to find fault where there isn't any. The creature can be wrong and the Creator never is.

Does the prodigal son story open our theology of the lost sheep metaphor or cause a disturbance?

I see no contradiction. Sheep wander off, and so why not mankind?