Good points and very good questions.
The broader picture here is that at the tactical command level we had some very capable generals who were strictly professional.
Generals like Yakub Khan already knew that East Pakistan was indefensible without the help of the local people who had turned against us. De-classified information now available shows that generals like Yakub Khan had already advised Yahya Khan as early as March 1971 that it would be impossible to hold East Pakistan, and via negotiations it should be let go. Henry Kissinger had briefed the US President Nixon about Yahya Khan wanting to vacate East Pakistan.
Before we ramble off into the "hows and why" this didn't happen, we must return to the topic of how Pakistan was able to deny its equipment to both Bangladesh and India. At the tactical level, Pakistan had 9 months to put in place a plan to disable any war materiel that would be lost.
The most telling aspect of Pakistan's resignation to the loss of East Pakistan was the fact that only twelve 40's era F-86 jets were left in East Pakistan for air defense, whereas there were dozen squadrons of F-6s, B-57s, Mirage 3, F-104s in West Pakistan as well as Sidewinder armed and radar linked F-86s. Heavy tanks, artillery, and engineer regiment were all in the West.,
It was clear as early as April that Pakistan was hunkering down to survive the Indian onslaught in the West. Pakistan hoped to save East Pakistan through foreign ( US or UN ) intervention diplomatic and military posturing on India, not through military action which was unfeasible.
We are a patriotic people at the ground level, and regardless of how flawed our leadership is, the person on ground knows his duty. So PIA ground maintenance crew at Tejgaon Airport in Dacca had plans to quickly destroy the instruments, tools, maintenance rigs, control tower communications equipment before fleeing, which they did .
The 12 F-86 Sabres were also disabled of which only 3 could be restored to a precarious flying condition.
If there are 45,000 combat troops and they are patriotic ,and not traitors, and if they are trained to destroy their weapons they WILL do it,
If each squad is assigned to destroy their own equipment it can be done in a very short time.
An artillery piece can be damaged by an overcharge in its barrel, and a tank can be disabled in minutes by fouling its fuel line with an additive ruining the engine, and later blasting its tracks off with an improvised charge.
A patrol boat can be sunk within minutes by a charge in its hull.
The Germans scuttled the pocket battleship 16000 MT Graf Spee in minutes after the Battle of the River Plate ( December 1939), rather than allow it to be captured. If you talk to veterans of the 1971 war now fast dying off, they will tell you how they simply pushed their trucks into the water off a bridge. Small arms are the easiest to destroy by taking a welding torch to the muzzle and plugging it or simply removing the bolt or trigger assembly and crushing it with a hammer or throwing it into the water. If each soldier is trained to destroy his personal weapon there isn't much left to be captured after surrender.
No army in the world allows capture of their equipment, even if they surrender themselves, and they know exactly how to destroy their equipment easily and quickly.
This is not to say that weapons are not captured when soldiers fighting to the end, are killed instantly in an air attack, or shelling and their warehouses or arms depots are overrun in a lightning campaign. This happened early during World War 2 at Dunkirk, or in France and Russia but even here the British were smart enough to disable a large portion of the equipment they left behind.
The fighting in Bangladesh was nowhere close to the intensity of Stalingrad or Berlin. General Niazi had a full 24-48 hours to surrender and in fact the situation had turned as early as 11th December itself. There was ample time for the soldiers to begin destroying their equipment.
But there is further evidence that very little fell into Indian or more importantly Bangladeshi hands.
Indian officers like Colonel Piyush Ghosh, and others themselves say Pakistan efficiently disabled or destroyed its equipment If India had captured large quantities of vehicles, trucks rest assured they would have released a whole movie on it. See this
video ( point 17:24) of equipment of surrendered by East Germany after reunification in the aftermath of the Cold War. East Germans could have destroyed their equipment but they did not.
When I visited Bangladesh in 2008, the BDR were still using Chinese supplied SKS rifles, from the 1940s and there were still SMLE 0.303 rifles with the police.
There were very few obsolete Chaffee light tanks in use in Bangladesh. Even these were disabled.
My conclusion is that the biggest assets Pakistan had were a large fleet of Boeing 707 B aircraft which maintained two flights linking Karachi, Dhaka, daily and some Fokker Friendship turboprop aircraft . These were recovered and returned to Pakistan.
India could keep General Niazi's Mercedes.
