Ok, hypothetical for you; lets say Ned had sent, along with Beric and Thoros and the rest, Loras, to apprehend the Mountain. Everything else happens as in OTL (They get ambushed, Beric is slain, but then Thoros brings him back, etc). How do the Tyrells react to this? How does Renly react? What are the long term consequences?

So, it depends on what precisely happens to Loras. 

• If Loras dies, the Tyrells go to war against the Lannisters faster than in OTL, although they probably won’t side with the Starks who Mace probably would blame for getting his son killed. If Renly still tries for the throne, and gets killed as in OTL, it’s probable that the Tyrells will pull back and later declare for Aegon (and against King Stannis) when he lands, especially if Aegon will marry Margaery. 
• If Loras is captured, it’s possible Tywin ransoms Loras to them for their support (possibly with Joffrey as sweetener), at which point the Lannisters probably run away with the war completely well before the Red Wedding, using the Tyrells as their battering ram. Renly probably negotiates for a pardon, but might well scheme to try to take out the Lannisters from within.
• If Loras gets away, you probably get a scenario a lot like the first, but without resentment toward the Starks. Possible that the Tyrells might side with the Starks post-Storm’s End, given that Loras will be quite vehement that you can’t trust someone who attacks the King’s banner to hold to any bargain, but equally likely that they pull back and later declare for Aegon.