Not just push the ruble up. It pushes the other currencies down.
Why do you think the euro is losing value and the fall sped up after gas for rubles scheme?
Most gas payments are likely being made in euros.
Euro has been losing value even prior to that because Fed has been hiking the rates way faster than the EU. So while the "gas for ruble" scheme is obviously affecting the currency movements, the primary driving force has been the reluctance of the EU to quickly raise the rates due to the fact that the European economy has been largely propelled by QE and negative rates to produce some growth.
If the Russians did the same with oil payments in dollars then dollar would likely collapse.
No, it wouldn't unless all other major exporters will also ditch the dollar as well - on a side note, I wonder whether the talks with SA and China about paying for Chinese oil imports with yuan are still ongoing. Plus dollar is used not only in the oil trade but almost everywhere, including all other commodities.
And yes the Russians can reduce the amount of exports to these sanctioning countries. Of course, the thing is, the Russians do not want to be the ones to break the contracts and pay penalties. So, of course, they won't unilaterally break contracts. But you will see them continue figuring out ways to stop incoming unreliable money flows from these countries.
I am exactly talking about the latter part - which is why I think that Russia will start limiting the resource exports as Gazprom has done with one of the gas pipes.
And in the short term, you can't just change all your customers and move all the logistics around just like that, this will take time.
No one saying that Russia should stop all the exports to the West, just limit them enough to cause more economic damage and help stabilize the ruble by limiting inflows of the unreliable currencies. Another way could be to demand hard currency instead, this money is way harder to track than those virtual transactions.