How fully does Updike draw the character of Sammy? What traits(admirable or otherwise) does Sammy show? Is he any less a hero for wanting the girls to notice his heroism? To what extent is he more thoroughly and fully portrayed than the doctor in "Godfather Death"?

"A&P" is told from Sammy's perspective, so the characterization is done indirectly. Readers have to infer things about Sammy through the way that he thinks, the things that he says, and the things that he does. For example, an argument could be made that Sammy has a strong moral compass. It's why he stands up to Lengel's mistreatment of the girls.

You didn't have to embarrass them.

Another trait that Sammy shows is that he is impetuous. Sammy probably could have told Lengel that he was being unfair to the girls without quitting. Instead, Sammy thinks he can make a bold statement that will impress the girls. Unfortunately, he doesn't think about the consequences of losing that job until after he actually quits the job.

"Sammy, you don't want to do this to your Mom and Dad," he tells me. It's true, I don't. But it seems to me that once you begin a gesture it's fatal not to go through with it.

The quote shows that Sammy realizes that his impetuous action is going to have consequences, but the quote also shows that Sammy is the kind of person that will see things through. He commits to his actions. This is an admirable quality in a person, and it shows that Sammy isn't the kind of person or friend that is going to go back on his word.
As for whether or not his "heroic" actions are less admirable because his goal is a reward, that is up to individual readers. If a key hero trait is that the hero is selfless, then that might hurt his heroic qualities. He is doing his hero action for a reward. Of course, his action could also be seen as quite heroic because he stands to lose a great deal if he fails.