Hi anony, (SPOILER ALERT)
  
1) …no…well Sinbad died in Magi at the end (the pic you probably saw was the moment he died the first time….yes he died twice), but it has been heavily implied that he’ll be back.
Ja’far just thought he was gonna die (the other pic you probably saw, but it happened in Sinbad no Bouken’s storyline), but he’s alive.

2) sinja is not canon, it’s doomed to be queerbait forever. It still has chances to be canon “offscreen” I guess.

3) Yes, Magi ended.
 
4) It’s confirmed it would be unlikely for Sinbad to fall in love overall. And no, he ended with nobody. BUT HE’LL END UP WITH JA’FAR WHEN HE’S BACK. At least they will spend the rest of their lives together as it should be.

4) Magi ended with Sinbad dead and Ja’far waiting for his return. Sinbad died regretting all the bad things he had done and wanting to return to his friends.

THEORY TIME:

But tbh, the moment I saw the last volume cover (below) I knew Sinbad was going to return and in not so many years. 

The fact Sinbad appears in his human form, with his crown, with no metal vessels and without another important piece of cloth (that magentaish purple jacket…thing?) suggests this is Sinbad’s final form in the story and not old Sinbad when he was king of Sindria (before final arc). 
Everyone else in the cover is shown as they would look at the end of the story, even tho not all of them showed up in the last chapter. For example Kouen is shown on a chair because he lost his limbs after Zagan and all the other djinns were released. We knew this was going to happen, but it’s never shown in the story. It’s  only on the cover.

However Sinbad’s last form in Magi was the one of a god. Drawing his spirit flying somewhere wouldn’t have been difficult.
But we got a new human form instead.
Which means that everyone on the cover appear in their final appearances, Sinbad included.

So either, Sinbad will come back really soon like @murphy-stoffelis​ pointed out to me some time ago. Or he will return in not so many years, like I thought after seeing this cover. And when he comes back, he’ll return as king of Sindria, hence the crown.
PLUS in the cover it totally looks like Pisti is celebrating Sin’s return (?)

However there is more: really awesome people wrote theories about his return and even tho their ideas come from different sources, they arrive to the same conclusion. Sinbad may come back in 7 years!

You can read them here: Theory 1 | Theory 2