Your correct in analyzing why the U.S. lost. They spent their time chasing shadows, and being paralyzed by fear of escalation. If you can't focus on the enemies center of gravity, and attack his most vulnerable point don't bother to fight the war, because your not serious. The whole war was a series of half way measures. We used massive force applied in a deliberately ineffective manor. We bombed targets worth less then the bombs used to destroy them. We chose targets because they didn't matter to the enemy, and refused to hit targets that would have caused them pain. We did that because we weren't trying to destroy the enemies means of resistance, but were sending messages. They thought tonnage dropped would show the enemy our will, in fact it showed them we weren't serious.Again, one only has to look at the bigger picture to see why the American chief of staff thought an invasion of any Laotian territory would be very risky and elicit a dangerous response: they're losing goodwill in the international stage, losing domestic acquiescence for the war, and the Chinese and Soviets are eyeing the situation as an opportunity.
You also seem to misunderstand the delicate situation Laos found itself in at the time:
-There was the Pathet Lao supported by Hanoi, which we know. While not still a numerical majority, they still exerted considerable influence over their target audience.
-There was a pro-western anti-communist faction under Phoumi Nosavan that only controlled the capital Vientiane, Luang Prabang, and the immediate surroundings. They were supplied from Thailand, but their influence over the eastern borderlands was downright nonexistent.
-There was a "neutralist" faction under Kong Le in the Plain of Jars that often veered between both sides in the grand stage. There was one thing they agreed upon, which was that the Lao nation should be kept whole. They were gradually worn down over the 60's and most of their splinters defected to and swelled the ranks of the Pathet Lao, if i'm not mistaken.
-Finally, there were minority groups such as the Hmong and the central highland tribes who were armed and supported by the United States but whose cultural exclusion made them relatively easy to spot and isolate.
The US did infiltrate and disrupt the HCMT through covert special ops formations who were airdropped in with generic indistinctive uniforms and untraceable firearms. They also sent out air raids across the area with the objective to observe parts of the trail, bomb any formations they could find, and destroy valuable targets such as fuel and ammo depots. There was an invasion of the panhandle in 1971 by ARVN troops with US support. Overall, all of these were costly mounting failures.
What you're proposing is a direct invasion of the Laotian Panhandle by the US using committed ground troops backed by air power to occupy, damage and dismantle the trail. An occupation of a border strip would not have sufficed, they would have to go all the way to the Mekong for their objectives to be achieved. We have to analyze why the US didn't carry that out despite the notion that it would have been a silver bullet and the fact that NV troops already occupied Laos:
While i do concede that this would have been an immediate blow to infiltration efforts by the NVA and VC into SV and Cambodian territory, it would, as mentioned, have risked an escalation of the war. Laos, not just the leftist guerrilla groups but quite a lot more of the whole nation, would have thoroughly allied with NV to resist what would have been a very clear act of aggression and violation of their sovereignty by the US. Congress would be guaranteed to be furious and restrict the president's legal ability to handle troops and equipment. The CIA would be concerned at seeing the uniformed ones trudging upon what was agreed to be their territory. The Soviets and Chinese would be eager to help and apply military pressure on other parts of the world to distract the Americans and amp up the armament of NV and Laos, nukes could have been involved... there's a lot that could go wrong, and all three of the war's presidents knew it. That's why they did not pursue the option of trespassing into the Laotian border. Justifying it would have been a nightmare.
We also lost because we weren't being honest with the American People. In the LBJ years they never trusted the people would support the war if they knew how hard victory would be. Even among themselves they could never even define what victory meant, or why we were there. For if the trumpet give an uncertain sound, who shall prepare himself to the battle?
They wanted the people at home to forget there was a war going on. They feared if they tried to convince the people to support the war they'd want to win it. That was what made the Korean War so unpopular, not that they were fighting it, but that they didn't win decisively. At the same time they didn't want the people to turn against the war. They wanted to keep the people in nether forward, or reverse, but in Neutral Gear. You can't drive out of a ditch in neutral.
By the time Nixon came into office a trumpet call of charge would be politically costly, the smart money was against it. So they sounded retreat, but not too quickly. Peace with honor. The North Vietnamese would have to bid their time, but it was coming. They sang "Time is on my side. Yes it is." Watergate was the final nail in South Vietnam's coffin. So yes if you go into a war taking council of your fears you will lose.