The fact that they now exactly the route it took and how long it spent in which airspace shows that it was noticed. Probably it was identified as a reconaissance craft and therefore no action was taken.
No, all of those are just guesses, including the trajectory and what it actually is. Croatian president was furious, they are just guessing that based on the producer of the thing
Even if you are correct(which you are not), you do realise it crashed into a Croatian capital without any reason, few tens of meteres left or right and it could have hit a residential building or something like that and innocent people could have been hurt?