You may indeed be right – I may have been a bit hasty in my assessment concerning Syria. But I still believe it to be plausible.

First of all, there is and was sufficient evidence at least not to dismiss Syrian involvement or responsibility. The Mehlis report, named after the first UN investigator, stated that “Maher al-Assad, Assef Shawkat, Hassan Khalil, Bahjat Suleyman and Jamil al-Sayyed” were behind the killing; the first one being the brother of the Syrian President, all else being senior Syrian and Lebanese officials. Some witnesses may have been unreliable at the time. But I suppose it still fits into a general theme. Syrian President Al-Assad supposedly threatened Hariri in 2004; former Syrian vice-president Khaddam even went on television to implicate Assad by stating that Assad had personally threatened Hariri shortly beforehand. Also, in the aftermath of the Hariri assassination, many other anti-Syrian figures and vocal critics of the Syrian presence in Lebanon were killed in bombings, which hints at a broader plan to weaken anti-Syrian elements in Lebanon.
So I think that the Syrian government benefited a fair bit from all those deaths. It may be implicated and have less political clout for the time being, but Lebanon was still weakened enough from their standpoint.

An involvement by both Israel or the US doesn’t strike me quite as obvious. It would have made some strategic sense for them at the time being – especially as Syria had a bad standing in the US at the time – but ultimately I don’t consider it as realistic as a Syrian involvement. The US were already quite stretched with their occupational roles elsewhere and I didn’t get any grasp of imminent NATO plans to occupy Lebanon?
Also, it reeks a bit of a conspiracy theory – given a prominent book by Jürgen Cain Külbel blaming Israeli and US involvement, which I have not read but seems to be perceived more as a political POV-style book than an accurate account of factual research.

Would love to see some additional elaboration on your points!

Which governments benefited from the assassination or at least thought they would? Hardly Syria who lost their foothold in Lebanon and risked an invasion by the then rather more gung-ho US government. There are two other fairly obvious candidates. First Israel whose expansionist plans for the region backed by the US Bush administration seemed to know no bounds. They above all had the technology and the organisation on the ground. Then there is the unmentionable candidate the United States itself who clearly had plans to manoeuvre NATO into some sort of occupying role in the Lebanon. Apart from a stack of other enemies, Hariri was opposed to the plans that both of these ruthless and expansionist operators had for the Lebanon. He did not want his country to become a battlefield as became possible and a reality after the assassination and the departure of the Syrians.