This is almost a hypothetical question. Historically the Soviet Union did not invade Western Europe and did not initiate a nuclear exchange. So you are basically asking what circumstances (which by definition did not arise) would have caused them to launch those attacks (which they did not launch).

The declared position of the Soviet Union was that it would not attack first. Sources available to historians do not contradict this, with the possible exception until about 1980 that the Soviets might have launched a strategic nuclear attack if they had evidence the United States was about to launch such an attack (preemption). The Soviet Union officially announced a "no first use of nuclear weapons" policy in 1982. (The United States has never had such a policy.)

That said, Soviet operational plans in both instances would have been offensive in orientation. Because there was no practical defensive response to a strategic nuclear attack, the response would have been what the Soviets called a "retaliatory-meeting strike" consisting of launching their missiles. And the Soviet operational plans for fighting in Europe were offensive in nature (fast-moving mechanized offensive into NATO territory) because the Soviets strongly believed that offense was the stronger form of warfare and might bring any war to a quick, favorable conclusion, whereas standing on the defensive would not.

I find particularly informative the notes and transcripts from a series of interviews of American and Soviet officials conducted during and just after the collapse of the Soviet Union. These suggest the following points relevant to your question:

1. The Soviet leadership believed nuclear war was not winnable under any circumstances. Some Soviet leaders, including Brezhnev, were personally very anxious about nuclear war.
2. Soviet strategic nuclear weapons procurement was driven by competition with and fear of the American nuclear arsenal. They did not want the Americans to get ahead but they also were not planning to attack. (We can see here that neither side was planning to attack but feared the other was.)
3. The Soviets did not believe nuclear escalation could be managed and therefore did not believe there was a meaningful distinction between tactical, theater, and strategic nuclear warfare.
4. Likewise the Soviets did not believe a conventional war in Europe was likely to be contained. They thought their prospects in such a war were better if nuclear and chemical weapons were not used but were pessimistic about this.

Thinking now about the canon of Western texts speculating on WWIII, we can see that they do not align with what the Soviets later said they were thinking. If we are to believe them, the Soviets were not going to attack because the correlation of forces favored them or because they felt they were on the verge of losing some advantage. Although the idea of launching an attack on Western Europe to maintain control of Eastern Europe (in the face of, say, democratic revolts or economy-driven unrest) is prominent in English-language "war scare" literature of the 1970s-80s, in fact a situation similar to that transpired in late 1980s with no war. It turned out the Soviet leadership was more willing to lose Eastern Europe than go to war. Neither do the Soviets seem to have had any inclination to participate in the escalation of a third world crisis to WWIII.

Ultimately, it's impossible to know what might have happened in different scenarios, but what the Soviets actually did when things did happen and what they said they would do both suggest they had no plan or policy of attacking.