Any attempt to put anything beyond some Spetsnaz teams from a sub into North America would be an utter disaster (even the sub insertion would be questionable, with SOSUS and SURTAS in operation by that time). The U.S. and Canada maintained significant air defenses that were designed to stop BOMBERS, any attempt to launch an airborne assult would be bloody & brief.
BTW: The best way to ensure that the U.S. would go all the way (including nukes) would be an attempt to assault the CONUS. That tells the Americans that this is a fight for survival. It has historically been a very bad thing to engage the U.S. in a fight for survival.
As far the fight in Europe - Without WMD's the Red Army gets crushed trying the Fulda Gap. The NATO force that destroyed Iraq in 1991 with a fraction of the forces present in Europe in '89, does the same, albeit with considerably higher losses, to the Red Army.
Almost certainly the Warsaw Pact forces, especially the Polish, Czech, and Hungarian units (which just happen to control the Soviet lines of communication) turn on the Soviets at first opprotunity, especially if the initial offensive bogged down.
By 1989 the Red Army couldn't handle the Chechens, much less NATO. The force, like the U.S. Army in '74-'75, but far worse, was hollow; with the additional problems of no money to maintain equipment or to train and extreme corruption at the senior NCO and officer levels. The Red Army would have been lucky to have two motor-rifle divisions worth of armor be able to meet the bell.