Why did the US use so much ordnance during the Vietnam War?

Question:
Is it physically possible for the following to be true, together: A) the US used all the above (and 7000000 tons of artillery and idk how many bullets/grenades) in a region of that size, in a certain timeframe, and B) the US did not indiscriminately devastate Vietnam/et. al.?

There was no "Vietnam" throughout American involvement in the Vietnam War. There was a North Vietnam which was backed by the Soviet Union. There was a South Vietnam which was allied with the United States. The vast majority of the war was fought in South Vietnam, not North Vietnam.

What you've touched on is one thing which made the Vietnam War so different from other wars the United States has fought and difficult for American Forces at the time. For much of the war the United States did not target North Vietnam, an important source of fighters and supplies for the war almost entirely fought in the south.

Vietnam War
By 1963, the North Vietnamese had sent 40,000 soldiers to fight in South Vietnam.[61]:16 North Vietnam was heavily backed by the USSR and the People's Republic of China. China also sent hundreds of PLA servicemen to North Vietnam to serve in air-defense and support roles.

This was because the United States didn't want to risk the war widening. North Vietnam was allied with the Soviet Union and shared a border with Communist China. This broad strategy was called "containment", and was pursued throughout the Cold War by the United States in many theatres. Its central premise was to resist the communists expanding by fighting for stalemate, not victory around the world. Under this strategy engaging the N. Vietnamese in the South was US policy which for many years during the conflict entirely ignored N. Vietnam. This containment strategy was designed to minimize the risk of a wider war and made a war of attrition like WW1 or WW2 where one side tries to destroy their enemies ability to conduct war impossible. (Destroy their enemy's economy, agriculture, factories, power grid, and transportation systems as was done in WWII, and not done in Vietnam). Additionally the US ability to target enemy troops in the south was nearly impossible because they were indistinguishable from the general population. The US could only react when their enemy chose to show themselves in the South. (Tet Offensive). What the US was left with was trying to target North Vietnamese forces as they traversed roads macheted through the jungle between North and South. Example: Ho Chi Minh Trail. These roads were used to infiltrate troops and war materials into the south. Targeting them wasn't a very precise operation as they were hard to find and once found easily moved. Airplanes flying over the jungle could not see these roads. The US tried many different tactics to find them - sensors, informants, human patrols, and even deforestation of the jungle.

One tool which was used to engage these supply lines was the B-52 bomber which carpet bombed areas where the enemy was believed to be conducting operations. A single B-52 carried 70,000 lbs payload and was used to target literally miles of jungle. Compare that aircraft to say a WWII era B-17 Flying Fortress which carried payloads of only 4,800 lbs and you begin to gain an explanation for why so many tons of munitions were used.

The combination of:

1. The time frame of the Vietnam War - the USA was involved in Vietnam for 20 years, vs. 4 years for WWII.
2. The characteristics of the war - the Vietnam war was fought (nearly) entirely in South Vietnam. WW2 was a global war, where Germany and Japan were only partial theaters. Japan only came under sustained bombing by the allies from June 1944 - August of 1945. Likewise the air war in Europe was fought across north Africa and across Europe from France to Moscow. WWII was just a different kind of war.
3. No good targets for most of the war due to political restrictions and the inability to find the enemy in the south; also the characteristics of the Vietnamese jungle.
4. The capabilities of the air force having grown significantly with a single B-52 aircraft able to carry more bombs than entire squadrons of WWII era bombers.

These are the answers to your questions.

low: what are the descriptive merits of the term ecocide, here, or, when does the word "war" apply here more than/instead of "atrocity"/"massacre"? (I can rework this question for the Philosophy Stack Exchange or what, if appropriate?)

Deforestation of Vietnam or "ecocide" as you say, was a war strategy. The North Vietnamese used the Jungle to shield their movements as they conveyed troops and supplies deep into the south to pursue the war. The US strategy to deforest or kill the jungle which shielded the North was designed to make the movements of these foot highways more visible and thus more engage-able. Only the United States didn't use B-52's and high explosive bombs to deforest the jungle. Rather the US used strong herbicides like 2,4,5-T and 2,4-D, known alternatively as Agent Orange, to engage the Vietnamese jungles which in theory would allow the US to more precisely target the trails.

A friend of mind toured the Khe Sanh Battle field a few years ago, and he described his tour guide kneeling down and lighting on fire a hand full of dirt. This demonstrated that the United States dropped so much C-4 and other munitions around the 6000 Marines besieged at Khe Sahn for 6 months in 1968 that the dirt around the battlefield will still catch fire even 45 years later.