While it obviously wasn't the original intent and the spelling was different in the script, there was a mention in TNG of a starbase Zendi, if you want to that actual Zendi could be "retcon'ed " to be Xindi. The two words sound identical.Plus you run into the issue of wondering why we never heard of the Xindi on the other shows.
No, not really. The multi-species Xindi could still be out there in the universe and not get mentioned.SURELY, we would've heard of them somewhere.
Not every species has to be a call back. Nothing would have made me happier than if after the first episode we never saw the Klingons again during the series.To me, it would've made more sense for it to be the Romulans just for the sake of canon.
Tuvok seemed more comfortable in his own skin, part of his (in-universe) advantage over Spock might have been he lacked Spock hybrid nature. Spock struggled to control his emotions, Tuvok usually didn't. Tuvok was also quite a bit older than Spock.This should be quite controversial: I think Tuvok was a better Vulcan character than Spock.
If the two, personally I like Spock better, YMMV.