Ha: "it gives credence to the Russian position in Syria"? You probably should probably stop thinking of this war as a proxy war, where the primary contestants are the US and Russia, who are competing with each other over who occupies the moral high ground.
The US initially demanded that Assad step down, because Obama's policy during the Arab Spring was to side with the peoples of the countries who were protesting against their dictators, rather than the dictators because he was hopeful that democracy would emerge, and America wants to be on the side of democracies. It's still US policy to demand that Assad steps down eventually (even if after an interim period, sometime after the civil war is resolved through a peace treaty), but there's growing consensus that that won't happen, because Assad will likely win. Meanwhile, Russian involvement in the war is costly to Russia, because by aligning with Alawite/Shi'ite Syrian regime and Shi'ite Iran, it's incensed Sunnis within Russia's own borders and around it. But Syria was historically a Soviet ally, so the US doesn't really have a particularly strong relationship with Syria. It doesn't have any specific interests that it needs achieve by involving itself in the war, except it has an interest in regional stability, and resolving the refugee crisis, which is negatively affecting Europe. Alternatively, Russia has various military assets that it didn't want to lose, which would happen if Assad is defeated.
Arming rebel groups isn't some sort of massive hypocrisy that it was critical for America to avoid, again, as if US foreign policy is organized around avoiding bad PR. It's a strategic blunder, for sure. But if you live in Syria, you probably care if you're personally affected by it negatively, and you probably don't if you aren't.
lol. The US may be a superpower, but it doesn't dictate foreign policy to its allies. It obviously annoyed Obama endlessly that they couldn't stop Israel from building settlements. But at the end of the day, countries have only little amounts of leverage they can use to nudge their allies. And it can be costly for the alliance if its pushed it too far. (Although Hillary's 10-month settlement freeze in 2010 was actually an impressive accomplishment, especially given that no other president/sec of state had been able to accomplish it. And she did while Netanyahu was prime minister, no less). The Saudis didn't need the Americans' "permission" to fund ISIS.
It's also not as much of a "big, serious issue" as you suggest. ISIS may seem to you (and to many) like a modern day Nazi regime -- the very embodiment of evil. But countries don't bring such moralistic terms into their calculations when making strategic decisions. Why is ISIS a problem? Because of security concerns (both the homeland and the security of allies) and regional balance of power issues.
Even from the standpoint of concerns about anti-Americanism, the spread of ISIS isn't an unambiguous problem. To many Sunnis in western Iraq (Sunnis being the majority in the parts of Iraq where ISIS took most of its territory), ISIS was greeted as liberators. The Sunnis there detested the overbearing influence of the Iraqi federal government, which is dominated by the Shiite majority in the east. The Sunnis are also generally the most embittered towards America as a whole, because they were the big losers from Saddam being ousted.
I mean by ally that he supplied the US with extensive intelligence resources about Islamic militants in Afghanistan that were collected by the Soviets during the Russia in the 80s, and gave permission to the US to build military bases in former Soviet republics in Central Asia.
Clinton and Bush were both publicly critical of the Russians over Chechnya. They both frequently angered Yeltsin/Putin by referring to the Chechnyan militants as separatists, which legitimized their efforts at independence (although they sometimes used the Russian terminology, and called them terrorists. But it was only occasional, and politically motivated).
But, yea, I love how in your mind it's still the fault of the US when Russia commits atrocities because we "let" them do it.
No, it's not a rule that the more the US get involved, the worse things get. ISIS has lost 25% of its territory because of the role US military advisors have played in organizing coalitions of fighters on the ground. As I said before, Russia has its own interests in Syria that have nothing to do with the US. Russia didn't get involved because the humanitarian situation got worse. It got involved because Assad was losing.
Oh, I thought you changed your mind about US interventionism! /s