I think he is in the current time with the AG party, using the Echo to look back to the battle at silvertear lake, maybe because something in this leve is related to the battle.
As we have seen in some quests involving the echo (ingame), some people can notice us when we walk around in the past.
In the gladiator story, we even fight and win against someone else so battles in those Echo visions are possible too.
That also explains why he survived the "fall" at the end of the battle - it was just a vision, he snapped out of it before he hit the ground.
Now for meeting up in the future - there are 2 possible reasons for this.
Either the 10 year ago party felt so attached to him from their very short fight together that they searched all of Eorzea for him for more than 10 years (unlikely) or they just met up by chance in our current timeline.
In my opinion, this option makes the most sense since with all the Garlean and Dalamud problems we have right now countless adventurers are gathering up for battles; especially the giant battle to end the world
It's not THAT unlikely to meet someone you might know.
Heck, they don't even have to remember that they have met before. It could just be fate that they team up on the battlefield, maybe because they are the last ones standing, maybe because their "party setup" was better that way.
If they were sent to the future together by Louisoix it just makes sense to stick together there as well, especially since we know the ARR timeline is even more dangerous.

As for the different jobs: that one should be obvious and you said it yourself already - armory system.
All the adventurers are using it and we know the characters in the different trailers ARE adventurers.

I'm gonna ignore the 2009 trailer because it was early footage, doesn't make any sense and was changed afterwards, which is basically saying "Ok forget about this, look at the newer version of the trailer."