Peep Show/WMG

Mark and Jez are demons from hell
Seriously, watch the series from the begining but with this in mind and it's so obvious. They are demons from Hell just trying to fit in here on Earth. Other such demons are Don Danbury, and The Gang.
 * Don't forget the Seinfeld crew.

Something horrible is going to happen to
Just going on the nature of the series.
 * It's already happened; it has to grow up with Mark and Sophie as his parents.

Sophie's
She just said it was because he is easier to manipulate and take advantage of.

The entire show is an extended flashback by Mark
He's explaining to his psychiatrist/prison guard/executioner why he murdered fifteen close associates in cold blood.

Mark and Jeremy are secretly Reality Warpers, but they don't know it.
The trouble is, they always expect, secretly, to fail in their endevours, and thus reality warps to meet their expectations.

Jeremy will end up rich and famous, Mark will end up poor and dependent on him
It's been played with in the series and seems to be one of Marks many many neuroses

When the series finally ends, both Mark and Jeremy will be happy.
Fairly obvious; considering the series is their perpetual unhappiness, when it eventually ends the writers will Pet the Dog and give both of them a happy ending.