I imagine Russia would. Not because they want to defend communism in Cuba, but because they would want to show that they can't be bullied by the US. We already saw an example of this with Russia doing naval exercises with Cuba just recently. I also can't imagine that the US would dare attack Chinese ships going to Cuba. As long as there are nations that can't be bullied by the US who are willing to trade with Cuba there's nothing the US can do.

This is why the financial system is such a big factor. When everybody was using SWIFT for international trade, the US didn't have to threaten military conflict. They could just do sanctions or cut countries out of SWIFT. That was enough of a deterrent to make trade with Cuba unappealing. Now that more and more trade is happening outside the dollar, that threat is no longer there.