Oddly, having conducted a Mk8C powered Excelsior for some while I found the forward facing carburettor to be no problem. You take one look and the first thought is that it will suck up every bit of road muck and water you come across but it was never an issue. I'm sure it must have swallowed some bits and bobs but it wasn't bothered by it.
As to why it pointed forward, again, one look and you saw how much a rear facing manifold and carb would interfere with the hand gear change mechanism with both of them fighting for the same space on the bike. Remember, this engine was designed in the early 1920's when single speed belt drive bikes were still sold new and today's foot gear change was still years in the future. Designers were still finding answers to things we don't even recognise as a problem today. I think this is why I find vintage bikes interesting and modern bikes dull.
Cheers,