I have no idea about the second question, but I think the ending implied that the roots of Gabriel's family (or the Shattenjaeger tradidion) can be traced to that Roman guy. There is a discussion between Gabriel and Mosely in which Gabriel mentions that he tried to find the origin of his tradition but he could only learn that it was very ancient; I think this conversation was meant to foreshadow this discovery.
My memories are a little blurry but I remember that in the conversation between Jesus and the soldier revealed that the soldier promises to him that he and his descendants will be defenders of the light (?) or something.

Yeah, the Roman guy may have been a Ritter (which I suppose means knight), but what happened to his sword? As I said, Gabriel's dagger didn't look much like it...

I always took it to mean the golden hilt was reforged into the Schattenjager talisman. After all, I'm sure it became quite conspicuous (and eventually illegal) to carry a sword around after Medieval times.

I just finished the series and I was thinking the same exact thing with the sword. Why is it different than the dagger? If the two are different, where is the sword?

Also, I was confused about what happened to Emil in the end.

And.....what happened to Grace? What did she write in that letter? Where did she go?

Ugh!!! This is why we need a GK4.

Ok, here's a little GK history.
Yes, "Knight" means "Ritter".
The Schattenjäger castle stands in Bavaria, Germany. the Ritters are a german family.
Still, for some reason, all the diaries are written in English (not just for the english game, but according to the story). Now why is that?

Here's the thing.
The Ritters somehow knew that one of their heirs would need the journals one day and that he would not speak German, so they wrote all the journals in English. Gabriel Knight is the main reason the Shadow Hunters exist at all.

The dying Jesus said "I'll need a service from one of your decendents".
He KNEW that 2000 years later there will be the problem with the baby which shares his blood. So he tookt the roman sword, transformed it into gold (he also says something about the golds protecting powers) and gave it to the roman, together with a duty. The Schattenjäger (shadow hunters) were founded this very moment. The swords golden hilt later was molten into the talisman. Sure, they hunted down witches and demons, but the REAL reason they exist is for Gabriel to be in France on exactly that day 2000 years later so he could save the baby.