Yugoslavia being against the USSR is plausible imo, especially comparing that two of its Communist neighbours (Romania and Albania) might also refuse to support the Soviets.
In the case of Albania, there's no question: from the early 1950s onward they were actively hostile to the USSR. (And, of course, their ability to actually invade anyone was close to nil.)
In the case of Romania, up until the mid-1960s they were a loyal member of the Warsaw Pact. (Of course, they had Soviet troops occupying them until 1958.) After that they became increasingly "neutral" under Ceausescu. By the late 1970s it was very unlikely that Romania would join a Pact offensive without being forced at gunpoint.
Of course, that might have been an issue! The Soviets installed their Fourth Army in what's now Moldova for exactly this reason -- to keep Ceausescu in line.
Although i must admit the threat of a an armored Soviet offensive from Hungary might convince the leadership of Yugoslavia to think otherwise.
Probably not. Yugoslav defense doctrine basically called for refighting WWII -- that is, retreating to the mountainous interior and unleashing large-scale partisan and guerrilla warfare. They didn't make any secret of this, either... they wanted to make invading Yugoslavia as unattractive as possible. The country had swallowed twenty Axis divisions, after all (even if a lot of them were second and third-rate garrison formations).
Also, note that Soviet resources were not unlimited. As noted above, they had to divert a bunch of divisions just to keep Romania in line. Diverting another army or two in a (probably fruitless) attempt to swing Yugoslavia would leave then noticeably less to work with in Central Europe.
Tito's Yugoslavia had very limited offensive capability, but would have been a PITA to invade and occupy. It's really hard to see why either side would have violated its neutrality.
Doug M.