One weapon only, that cool?

It looks reminiscent of The Force Unleashed games.  Those games played amazingly, so I'm all on board with only having a lightsaber to work with.  Hopefully they have some good combos to unlock like in Force Unleashed as well.

Unlikely. We are a lost Jedi. So it is unlikely we know how to craft a light sabre. But I would not be surprised if we have to use vibroblade. Since kal looks like a clumsy clutz.

so not be surprised if a large part of game is finding it 

The Force Unleashed combat I found was really bad. It didn't really have much skill and was more about just mashing buttons.

Jedi Knight on the other hand had really decent lightsaber combat.

From the videos it looks like Fallen Order will require more skill and you will have to plan your attack (I hope).

The problem with games with lightsaber type combat sequences, it can be difficult to do right, especially saber locks and parrying, it can be difficult to do those unless it is entirely script phases.

I honestly found the scripted events in Force Unleashed, the closest we will ever get to lightsaber combat, while the Jedi Knight was not so good at it, from experience, parrying was not a thing in that game, unless it was a button mashing event, which is pretty much scripted to require a button mashing event to win.