Isn’t there a Free Syria Army that is backed by the West and is allied with a Syrian National Council of some kind, whose general Idriss recently evacuated from Syria to Turkey?

Nope, what has been the FSA is finished. The notion that Idriss recently fled from Syria to Turkey is AFAIK incorrect. For safety reasons he always worked out of Turkey, though his FSA operated an HQ and a weapons and ammo store in Babisqa near the Bab Al Hawa crossing some hundred meters inside Syria. What happened at that day in December was, that units of the Islamic Front took over that HQ and plundered the weapon stores of the FSA, while the FSA guys guarding it fled to Turkey:

“The FSA warehouses are now empty after they were occupied and their contents were stolen, including Salim Idris’s own office,” the FSA officer said. He claimed that Commander Abu Al-Nur of Ahrar Al-Sham led the operation. He said that the Islamic Front raided a total of ten warehouses belonging to the Western-backed umbrella group and seized a significant arsenal of weaponry, including 2,000 AK-47 rifles, 1,000 assorted arms—including M79 Osa rocket launchers, rocket-propelled grenades, and 14.5mm heavy machine guns—in addition to more than 200 tons of ammunition. At least 100 FSA military vehicles were also taken in the attack. (Source)

Before that Islamic State in Iraq & Sham (ISIS) defeated the FSA in Aazaz, the entry town into Syria across the Bab Al Salama crossing, and most FSA fighting groups pledged allegiance to the Islamic Front (IF). Since the FSA has no HQ and close to zero fighters anymore, it's no major group anymore. In the south of Syria there are seen some FSA flags, however it's to a large part just masquerade for the Nusra Front:

“The FSA and Al Nusra join together for operations but they have an agreement to let the FSA lead for public reasons, because they don’t want to frighten Jordan or the West,” said an activist who works with opposition groups in Deraa. “Operations that were really carried out by Al Nusra are publicly presented by the FSA as their own,” he said.(Source)

Most, if not all, of the few remaing FSA guys recently formed a group called "Syrian Rebels Front" (SRF) - which ist headed by Jamal Marouf. They pledge alligiance to Idriss and the SNC, but they are not a major group, just controlling a few villages in the Idlib province near Kafranbel and a little bit in the Aleppo province. Recently the SRF coopted some IF and Nusra guys to start a war against ISIS. In their villages in Idlib the SRF won, and in the Eastern part of Aleppo city some criminal guys persectued by ISIS managed to force ISIS to hand the town administration over to the Nusra Front, but in East of Aleppo province, in Raqqa & in Deir Ezzor those SRF allies who tried to challenge ISIS suffered an ugly bloody defeat. Supporters of the SRF claim the defeat of the IF against ISIS is because the IF doesn't fight using "the SRF method : no survivors, no mercy." It tells you a lot if SRF supporters claim IF guys are too soft. The Islamic Front IF has among seven components two main components: Ahrar Al Sham and the Islam Army. Ahrar Al Sham leader Hassan Abboud is a self-confessed independent Al Qaeda component fighting for an Islamic State as advised by Ayman Al Zawahiri's emissary Abu Khaled Al Suri, while Zahran Allosh, the leader of the Islam Army, fights for cleansing Syria from Shiites and Allavites and building an Islamic caliphate. The Nusra Front is Al Qaeda's official Syrian branch, fighting for an Islamic caliphate in Sham. ISIS is Al Qaeda's official Iraqi branch which expanded into Syria in the spring of 2013 fighting for a global caliphate. These threee, IF, Nusra and ISIS, are, besides the YPG, which is rather unproblematic, the major armed groups currently fighting against the government of Syria. Al of these fight with car bombs, the latter two also with suicide bombers, and all of them apply a strong version of Sharia including flogging people, while the latter two also publicly execute and behead people as punishment for what they deem criminality. The above descripton of the major armed groups operating in Syria may give a hint why western powers currently rush to Geneva to make some kind of peace deal with the Syrian Arab Army, so they can support the Syrian Army fighting Al Qaeda.