To provide some context, this is what the street level looks like:

 

 

 

 

The neighbour's driveway is slightly higher and not inline with the flow off the street, difficult to see in the photo, so currently doesn't get anything coming down it. The problem with breaking the flow up too much before it hits the first drain is that some of it will go left, where the sandbags are to prevent it coming down to the house, or possibly right, down the neighbour's driveway. The only thing worse than water coming down the driveway would be water not coming down the driveway.

 

 

The grating itself doesn't cover the entire width, just wide enough to bridge the catchment area for foot traffic, the updated diagram shows it a bit better. The thinking behind having the front slightly elevated was that if it's at the level of the water flow then leaves will get washed onto the top of it and provide an impermeable cover that directs water over the top rather than down into the catchment area.

 

 

The front drain does actually catch some of the flow, it's scoured completely clean each time there's overflow indicating that a fair bit of water goes down there, it's just nowhere near sufficient to catch all of it.