Yes, the other reason I bought the Ruroc was impact protection (I'm just getting going on rails and I currently still have all my teeth!).
Yes, I think the chin guard would provide protection, so there is that consideration.
As for sealing out the elements, I did say that I was not successful getting the chin guard to click in properly (it seems), so I'm still going to try it. But there is still that issue of removing it for whatever reason and then reconnecting it.
Yes, the moto helmet in your picture would be ideal. The OSBE helmet works that way, but without the chin guard.