A bit of speculation after looikng at the Map:
There is only one route from Tel Aviv to Russia across the sea and that is via KUGOS.
( I guess the Bulgaria connection is totaly out of the way and would make no sense for a Russian Airline to go and refuel there anyway..)
KUGOS northbound is controlled by Simferopol, a joint Ukrainian -Russina facility If I am not mistaken. Therefore this centre , if any, would know about the NOTAMS and the Military excercise.
The fact that another a/c below the Tu154 saw the explosion would confirm that both were in same area and that is most likely the airway , or an airway crossing, indicating they were not astray in a Mil area.
The Ukrainian Navy would be cautious is firing long range high altitude missiles that could end up outside of their FIR ( and they claimed they only fired short range missiles anyway)
The adjacent FIRS are ot that friendly and some are full of muslims activists at war with both Russia and unlikely to love Isrealis..
The Sibir flight is a regular one ...
If indeed the US intelligence confirms the Tu154 was downed by a SA-5 misile , and the Ukrainian Navy did not fire any, then someone else might have....
Just a thought, nothing else.
Speculation at this stage, but this is a rumour network after all....