Why was the US/NATO convinced the SOVIET Army would invade Europe by pouring through the Fulda Gap?
Tactically for armored warfare it seems rushing across the flat terrain of Lower Saxony, driving a wedge between the US and UK sectors, as well as seizing or isolating Bremerhaven and potentially the major ports in the Netherlands would have been far more logically sound.
Now that the Warsaw Pact has been abandoned for more than 20 years, do we have any knowledge of their actual war plans or tactical/strategic objectives had they invaded?
They would have done both. The entire Warsaw Pact force wouldn't have gone through Fulda, a large part would have swept the North German Plain as you suggest. The drive through the Gap was intended to strike at the American military infrastructure in Germany, most of which was located in the Frankfurt/Middle Rhine area (such as the Rhein-Main air base and much of the pre-positioned equipment for American reinforcements.) If the Pact could hamstring the American military, the remaining NATO allies would theoretically have been able to be disposed of at the Pact's convenience.
The terrain of Southern Germany tends to funnel an attacker who is aiming for France/England. There are only so many lines of approach for a large mechanised army.