Several coup happened in Pakistan that failed. 3 of them are well known.
1: Coup against Liaquat Ali Khan that failed. Many got imprisoned, including Faiz Ahmed Faiz.
2: A plot was made to kill Zia-ul-Haq, if successful than coup would have happened. Plot got foiled.
3: Coup against BB government in 1995 under Major Gen Zaheer-ul-Islam, where many young officers were involved. Coup was foiled.
[Coup also happened against Yahya Khan. Most were young officers and Yahya khan was also willing to hand over the power. Hence, it seems, all happened in peace].
Actually, many other minor coups happened in Pakistan that got foiled without making much news.
As for legitimate government … fact is that, there was/is no legitimate government in Pakistan.
1: Bhutto was not elected Prime Minister. He became Prime Minister by default. Election happened all over Pakistan, and his party got 80 out of 313 seats in National Assembly. Mujeeb had 168 seats and he deserved to be Prime Minister. Bhutto declared that Pakistan consist of two parts, one in east and one in west, and he would sit in parliament on parity, where he should be representing west and Mujeeb representing east. Modalities should be agreed before calling the parliament. Mujeeb objected, and military operation started.
Country got divided. After country got divided, 1970 election became null and void. Anyhow, once Yahya resigned, Bhutto was made President and Chief Martial Law Administrator by armed forces. Obviously, he could not have become Prime Minister, as election became defunct after Pakistan break-up.
Once constitution was made … remember, even constitution was not legally made, as those who made constitution, they were in minority of 1971 elected assembly (of whole Pakistan). For legitimacy of the constitution, new election was necessary and than only new constitution could have got made.
Anyhow, once constitution was made, still no election was held, and Bhutto illegally became Prime Minister.
After 1977 election, Bhutto again became Prime Minister, but it was accepted by most parties that 1977 election was massively rigged by Bhutto, hence all result of 1977 election is illegal. People came out on road, and Bhutto imposed Martial law on almost all big and small cities of Pakistan. In the end, it is claimed that even Bhutto agreed to make the election null and void, and to call new election. Thus, Bhutto being PM on 1977 election was illegal, so was the tenor of Bhutto.
2: As for BB and NS being Prime Minister, since none of the two fulfil constitutional requirement of article 62 and 63, their consisting election and holding office is illegal.
Hence, one can say that none of the coups that happened against illegal governments of Bhutto, BB or NS, were illegal.
Even today, if coup happens against NS government, that would not be illegal, as it should be considered as state institution removing an illegal government, because NS do not fulfil constitutional article 62 and 63 to contest election, thus he is illegally occupying PM post, and it is duty of state institutions to remove him (if EC did not do the job, that does not mean, army could not).
You should see things this way: Suppose a person is employed as medical doctor in hospital, but his employment is based on fake degree, than to remove such person from seat of medical doctor, even by force, is not illegal, because he is not medical doctor.
Same way, a person sitting on PM seat when that person is not even qualified to contest election or won election through rigging, than to remove that person from PM seat, even by force, is not illegal, rather just.
