Spoilers here too--When Sweeney accidentally gives the golden coin to Shadow, it likely was a mistake encouraged by Wednesday, as we later saw that Wednesday used the appearance of error often in his con jobs. Hiring Sweeney may have been only a pretext on Wednesday's part, to gain Shadow some protection at no cost to himself. Of course, Shadow was set on his own mythic path, and thus wasn't perfectly steered by Wednesday's intent, as shown when he threw his coin into Laura's grave. It isn't clear if Wednesday predicted that Zorya Polunochnaya would give her protection to Shadow, but he certainly wished to capitalize on it, and so warned Shadow to keep the silver coin.
Later, recognizing that Shadow was gaining some power of his own and becoming noticed as a result, Wednesday parked Shadow in Lakeside specifically to keep him out of sight of other gods and entities. Warning Shadow not to practice Sweeney's trick was one way to keep Shadow from recognizing his own nature and growing as a mythic hero, and thus becoming more visible to other mythic figures.
There are a lot of facets to what happens in American Gods, as Gaiman employs a lot of echoes and symbolism. Here, just as the magic of American Gods is both real and illusion, so too are the magic tricks Shadow is teaching himself, once he receives the magical coins.
As to being the King of America, that was a role that Odin was reserving for himself, not for Shadow. And Low Key, by teaching Shadow coin tricks in prison, was showing himself to still be a doomed trickster, giving Shadow the first nudge along a path that led to his and Odin's downfall.