My Name Is Earl/WMG

Karma affects people's personality
Compare Earl's personality during the pilot, he's selfish and greedy and is only doing good things because he thinks he will be rewarded, but by the end he does the good things because he's become a good person. And look at Randy, at first he's a bit slow, very crass and ill mannered. By the end, he's basically become a dimwitted, gentle giant.

Earl Jr really is Darnell's son
The simplest excuse is that Darnell's toothbrush was new, and had not been used yet.

Earl Jr is EARL'S son
But do the genetics work?

Earl died after being hit by the car
Earl is in purgatory -- a rather kind purgatory which, seamlessly simulating his real life, allows him to literally atone for every sin he's ever committed. This would explain why the hundred thousand dollars with which he funds his list projects (and pays rent, and buys food for two, and so forth) never seems to decrease. In this theory, there is a tenuous possibility that Catalina is some kind of angel.
 * It's in line with Dante: people in the purgatorium ascend to heaven only after they've repented. In Dante's account, ascension is honour-based. Prison was the second terrace; coma is the third.
 * It's shown that the money he won did decrease: in the prison plotline of Season 3, he used the very last of his money to fund a prom for the prison (yes, yes, ). But this would also be in line with your theory, since a few episodes later (well, the season finale), he gets almost the same amount of money back.
 * Earl seems to live a pretty cheap lifestyle, it wouldn't surprise me that one of the few things he's good at is making modest winnings go a long way.
 * Earl is also a man of simple tastes. A bowl of 'the stuff that [Crabman] couldn't legally put in the crab cakes', a cold Heisler Gold and his friends are all he wants and really needs to be happy.
 * If this WMG was true, then this would be the biggest Kick the Dog Tear Jerker moment in the entire show. Its the ultimate Shaggy Dog Story from depression!

The cop from "Cost Dad the Election" was working against Earl's dad and voted for the rich golfing guy
Seems odd that he'd send in a tape of Earl getting arrested for disorderly conduct twice for no stated reason other then to get the people of Camden to vote for the golf guy making the airplanes go over the community part the main cast and Earl's parents live in, doesn't it?.

At the end of "Bad Karma" Randy goes insane
His line at the end, "It's my show now" wasn't him breaking the fourth wall, It's him not recognizing fact from fiction anymore as one of the people he loved just died horribly and he can't handle life anymore.

Earl is narrating from beyond the grave
This is similar to the first WMG on this page, but different enough that I'll give it its own heading. Earl didn't die in his initial accident, but has died by the time he narrates the show. The giveaway is in the first season episode "Boogeyman." Earl theorizes that, after death, people can watch their lives like a TV show. Earl and Alby (the kid Earl's helping out this episode) then greet their future selves. When Alby does it, he faces the camera and says "Hi, Earl." But when it's Earl's turn, he looks off to one side. The implication is that we're seeing this from the dead Earl's perspective.

The DNA test was tainted
Think about it. Earl alerted Little Chubby that he had his DNA, and the results were delivered by Catalina. The Chubby's also have ties to nearly every business in Camden Country. It is VERY likely that he altered the results.
 * Also, even without the Chubby connection, this is Camden we're talking about. If they went out of Camden to a competent geneticist, doctor, or talk show host, it would likely prove that Earl Jr. is Darnell's, since Joy apparently never cheated on him except that one time with Earl, but the jury is out on Dodge.