A lot of what is stated is because we enjoy the benefit of hindsight. Yes, the establishment has acted extra-constitutionally in the past, no denying that, however not everything is wrong/extra-constitutional or a case of the establishment going against majority. Case in point, the PTI's ouster. The establishment does what it thinks is best for Pakistan, the same as the policies of any of the political parties when in power. Do these parties always have absolute mandate when they push their policies through? No, so just like that is an imperfection, so are the calls made by the establishment.
Army, primarily of West Pakistani origin, was cajoled to ensure West retained its leadership of the Federation of Pakistan over the East. Who cajoled the military? One should ask that question as the politicians in the West were not open/allowing MuR to form government. ZAB was hell-bent against it as were countless other Punjab based politicians. One should look up the newspaper reports from the time. I did that in microfiche format in the late 90s at the Library of Congress. Countless leaders spoke out against it and were asking the military to take action for the sake of Pakistan's survival (I was reminded of the same during the Lal Masjid Operation days when everyone was asking why the Army is not taking action only to leave the blame on the army after action was taken).
Army did not kill him. You should ask the family of Nawab Muhammad Ahmed Khan Kasuri about what transpired there. A murder was committed of this gentleman and the criminal case implicated ZAB as having been behind that case. Yes, Zia did approve his death sentence but a civilian court prosecuted his case lodged (in 1974) before Zia Ul Haq took power in 1978. So these facts should be kept in mind before assigning blame to the army. A mercy petition on the death sentence went to the President of Pakistan (Zia) and he rejected it. That was the only part that Zia played in it. No other army personnel were ever involved in either framing the case or prosecution etc.
Yes, agreed that establishment supported NS because the Army high command did not trust BB as she was deemed too close to the West and anti-army then and just like it used ISI to support IK in 2017-18, similarly it tried to influence national elections. However, NS was as popular as BB (the usual Punjab vs. Sind split) so it wasn't a case of the establishment supporting a minority leader in NS.
Army preserved its own leadership which was not being allowed to land after the then CoAS visiting SL. There were a lot of other misalignments too including Kargil, tussle with the judiciary, corruption cases etc. and the ongoing, dire situation with the economy that resulted in the establishment taking over the government. I don't condone this but also feel that in those times, the public came out thanking the military for booting NS out of office given the mess that had existed.
I think it is a misinterpretation to see establishment as supporting PDM. There has been a reset till next elections. PTI took certain support for granted to stay in power. That support was withdrawn. However there is no support for PDM nor PTI currently. Imran may be the most popular leader for a certain segment of the society, but that does not translate into PTI winning absolute majority at the polls. On top of that, IK/PTI made plenty of decisions that hurt Pakistan as well and we are living through these issues now. The same goes for all involved including the establishment in the past.