1. Short thread:

Photo 1 is the flight path of Mahan Air Flight 1152.

Photo 2 is MEA Flight 403.

Photo 3 is Qatar Airways Flight 416.

They all used the same Syrian airspace near the US Al-Tanf airbase in the last few days. Only the Mahan flight was intercepted by an F-15.

2. The Mahan flight was heading to Damascus, rather than Beirut... Did it begin its decent nearer to Al-Tanf?

No. Altitude when passing near Al-Tanf does not appear to have been the key factor.

IRM1152: ~10,350m
MEA403: ~10,350m
QTR416: ~11,580m

3. Some airlines flying to Beirut from the east totally avoid Syria, going around Israel and approaching Beirut via the Mediterranean.

Still, the airspace near Al-Tanf is regularly used by civilian flights + IRM1152 was not on an unusual flight path or at an unusual altitude.

4. The pilot and his ground control must have known it was a Mahan Air flight before the interception....

Not sure I see why an F-15 needed to perform a "standard visual inspection" to “to ensure the safety of coalition personnel at al-Tanf garrison.”

5. Finally, a point on why people fly Mahan Air, which has been branded a "Terror Airline."

If you're Iranian and you want to travel to see loved ones, conduct business, pursue education... Mahan may be the only affordable/practical option.

It's as simple as that.