Let's say that Rafik Hariri is assassinated in a more surgical way and one in which Syria maintains plausible deniability. Maybe one of those ricin-laced umbrella things the Bulgarians got Georgi Markov with or that succinylcholine stuff, Tom Clancy's favorite plot device. Nobody else except him is killed.
I think this would have been enough to butterfly or at least substantially delay the Cedar Revolution. While a lot of people resented the Syrian occupation and pro-Syrian government before that, staging a massive terrorist style bombing on the streets of Beirut that killed tons of innocent people was the main event that turned people against them. That will destroy even the closest alliance instantly.
So Syria is still in Lebanon, there's a pro-Syrian government, and the 2006 Lebanon War still happens. The core events that caused it are still there; Sharon was at the end of his life after decades of obesity, smoking, and general abuse of his body, Olmert becomes PM, Hezbollah still wants Samir Kuntar and his compatriots released and will launch Operation Truthful Promise, etc.
What does that war look like and what happens? Do the Lebanese Armed Forces join the war with a pro-Syria government at the helm? Does Syria get involved wholesale with Hezbollah?