Its all quite precarious.
Israel would never accept refugees because it's Israel. Enough said. Saudi Arabia probably wouldn't turn them away, but the trek across western Iraq is a long dead zone. They would likely encounter persecution in Sunni Saudi Arabia anyway since they are the anti-thesis of everything Shia. Jordan is cut off from Syrian Government forces. Turkey is closest. By foot, train or car that leaves Turkey or Lebanon. I think the fact that most Syrian refugees are heading away from the direction of Damascus -their country's capital- instead of toward it, answers most of our questions. They consider themselves as much a refugee from Assad as they do from any of the opposition groups in the country. This is a revealing question to ask refugees. Whom in Syria displaced you? From whom are you running away from?
I'm not sure the Syrian government is legitimate as you state it is. Careful where you tread or you'll support military coups, dictators and authoritarianism. I'm not convinced that it is accurate to state that your identified aggressor nations have annexed Syrian territory. US/NATO/Israel haven't attacked Syrian military forces except in isolated incidents. Yes, yes, the formula is ISIL + al Nusra = USA + NATO and or Israel. Let's call this the proxy formula. But there are other formulas also. I know you probably think this is naïve, but the sheer reliance on the proxy theory is potentially naïve. If they are proxies, would we actively try to destroy them? Maybe we would. There is record of American airstrikes in the region in question. If the Americans weren't attacking Syrian government forces or ISIL, than who were the Americans attacking? As part of Operation Inherent Resolve, the US and Coalition claims to have attacked, "As of 3:59 p.m. EST Mar. 1, the U.S. and coalition have conducted a total of 10,715 strikes (7,159 Iraq / 3,556 Syria)." The proxy formula may not be coherent if these were real strikes. Therefore are we to contend that these are fake airstrikes? The counter to this argument, is that the U.S. attacked Al Qaeda even though the U.S. supported Al Qaeda.

Here is another formula. Western Syria civil war + power vacuum in Northern Iraq after US withdraw = reactionary radicalized opportunists = ISIL + al Nusra. Let's call this the power vacuum formula.
Both formulas are valid, but one may be more probable than the other. I'm not seeing the case for one formula over the other at this time, whereas you and most on this board, have clearly picked a side.