What is "houri heaven"?
The houris are the name for the fair skinned ever-pleasing 72 virgins that a Muslim male purportedly receives in heaven, as reward for a lifetime of devotion. Although to be fair, I've heard recently that "houri" can also be translated as "raisin".
Are you saying that christians&jews get a chance to repent to Allah on J-Day, thereby going to Allah's heaven
Yes.
while already practicing Muslims aren't allowed to repent on J-Day?
If Islam was the One True Religion (TM), then Muslims wouldn't need to repent, surely?
PE