Not a spinoff, canon. They just decided to make an OVA instead of a new season because that was literally all that was left of the manga at the time; most likely because Hiroe Rei was busy drawing fanart/porn of the Fate series *********
No, there's a new arc that's ongoing. Unfortunately it's not enough for a new season either because it's only one volume (and a half) and Black Lagoon is on hiatus again.
Also Roberta's Blood Trail is based on an arc that took up three and a half volumes but they couldn't make a single season out of it.
No, it'd be better to compare it to old action movies, like Rambo or Terminator so yeah, it's awesome, but not all that similar to Fate/Zero .
Stuff that's related to Fate/Zero would be the other Fate stuff would not personally recommend, but they're from the same universe or most of the other stuff from Urobutcher which I would recommend like Puella Magi Madoka Magica or Psycho-Pass.