Had Uthman bin Affan no children that Muawiya became his heir? If Uthman bin Affan did have children who were his true heirs then it was duty of Uthman's children to go to the Muslim Caliph of their time to avenge their father's blood as only Muslim Caliph has authority to avenge their father's blood.
There is no evidence which negates that, the ones who deserved to demand the Qisas of Uthman(as), didn't demand that from Ali(as). But as we know that Ali(as) was delaying that matter due to which people the problem started.
What proves the fact that, Muawiya(as) had the right to demand for Qisas of Uthman's(as) Murder is that a large number of Sahaba demanded it from Ali(as), and Ibn Abbas said this:
واعلم أن على القول الأول والثاني ظهر أن المقتول وولي دمه يكونان منصورين من عند الله تعالى وعن ابن عباس رضي الله عنهما أنه قال : قلت لعلي بن أبي طالب عليه السلام وأيم الله ليظهرن عليكم ابن أبي سفيان ، لأن الله تعالى يقول : { وَمَن قُتِلَ مَظْلُومًا فَقَدْ جَعَلْنَا لِوَلِيّهِ سلطانا } وقال الحسن : والله ما نصر معاوية على علي عليه السلام إلا بقول الله تعالى : { وَمَن قُتِلَ مَظْلُومًا فَقَدْ جَعَلْنَا لِوَلِيّهِ سلطانا } ، والله أعلم
For the tafseer of this verse Whoso is slain wrongfully, We have given power unto his heir, quran(17:33) Imam fakruddin razi writes , It is reported from hz ibn abbas(ra): I said to hz ali(ra) , by Allah (Muawiya)ibn abu sufyan(ra) will surely overcome you because Allah said Whoso is slain wrongfully, We have given power unto his heir. And hasan basri(ra) said that because of this verse Hz muawiya(ra) was successful compared to hz ali(ra). Wallahu alam.[ Tafseer al kabeer vol 4, page 397)]
There can be NO Ijtihad where there exists clear commandments in Quran and Hadiths. A person cannot go against clear injunctions laid down in Quran and Hadiths by using his own Ijtihad. For e.g. I cannot pray 5 rakats of Fajr Salat based on my ijtihad or consider four sips of Wine to be halal based on my ijtihad.
True as for the example you gave for Salat, but there are other cases where there is scope of ijtihad, and the matter of dispute between Ali(as) and Muawiya(as) was one of them. The learned scholars that's why counted it to be ijtihadi issue, so if a commoner who doesn't have basic Islamic knowledge can't grasp it, then his objection holds no weight. I can give you examples where Ali(as) directly heard the command of Prophet(SAWS) yet made his ijtihad, because he knew there was scope for it in that matter.
Usamah bin Zaid (r.a) killed a man in ignorance (he thought that a person wasn't Muslim in true sense as he professed Tawhid by his tongue but never believed in Tawhid by his heart). When Usamah (r.a) was rebuked by Prophet (s.a.w.w) he (r.a) felt regret for his (r.a) action and promised never to strike anyone who professed Tawhid.
He misjudged that person and made an incorrect ijtihad.
But in Battle of Jamal & Siffin, believers were not killed in ignorance, unintentionally or by mistake.
These battles were itself due to mistake in understanding.
So I guess there is possibility of Shaitaan being saved as there is clear hadith that any person who does not worship Ghair-ALLAH will be in Paradise.
You are repeating the same mistake again, trying to take general text and applying on individuals, your case is same to a person who declares a person Jannati just because he offered two units of Nafil prayer.
What mistake you repeated did in this case is that, you tried to apply a general text over Shaitaan, while there is Khas evidence present that he will be sent to hell.
Allah says in Quran:
That I will fill Hell with you (Iblees (Satan)) and those of them (mankind) that follow you, together."(Quran 38:85).
For the sake of argument, if it is to be believed that there exists khaas reports of Muawiya entering in Jannah... then what about his troops who killed the believers? Does there exist khaas reports of them also entering in Jannah after killing the believers in Battle of Siffeen?
As I told you, we should refrain from making judgements over people based on general reports, and when there is no Khaas(specific) evidence. Even though there are weak reports which state that both will be in paradise, that makes me stick to the Ijmai Sunni view of maintaining silence and not commenting over these matters.
وعن يزيد بن الأصم قال : لما وقع الصلح بين علي ومعاوية ، خرج علي فمشى في قتلاه فقال : هؤلاء في الجنة ثم خرج إلى قتلى معاوية فقال : هؤلاء في الجنة ، وليصير الأمر إلي وإلى معاوية
Imam Ibn Abi Shaybah narrates in his Musanaf, that after the treaty between Ali and Mu’awiya [May Allah be pleased with them], Ali walked between those who died from his party and saying: “Those are in Jannah”, then he walked to those side of those who died from the side of Mu’awiya and said: “And those are in Jannah [Musanaf ibn abi shaybah
Also I would like to bring it to your attention some reports, since you even brought upon the matter of Jamal too, and you even asked what about the soldiers in the army, and I can't comment on each individual, but for those about whom there is Khaas reports then see these.
بشر قاتل ابن صفية بالنار سمعت رسول الله صلى الله عليه و سلم يقول : لكل نبي حواري و إن حواري الزبير
هذه الأحاديث صحيحة عن أمير المؤمنين علي و إن لم يخرجاه بهذه الأسانيد
تعليق الذهبي قي التلخيص : هذه أحاديث صحاح
Ali bin Abi talib (RA) said ‘Give the news of hellfire to the one who killed Ibn Safiya ( al-Zubair bin al-Awwam RA) because I heard the Prophet(saw) saying :’ Every prophet used to have a Hawari (i.e. disciple who supports him), and my Hawari(i.e disciple who supports me) is Az-Zubair bin Al-’Awwam.
It was narrated by Imam Ahmad in his “Fadailu sahaba” (#1057) narrated that Ali said: “I hope that I, Uthman, Talha and Zubair from people regarding whom Allah said: “(15:47) And We shall remove from their breasts all spite that they had (in the world against one another due to misunderstanding). They shall be brothers seated on couches face to face (in Paradise).” Then be said: “Who are they if not us? Who are they if not us?” And he kept repeating it until the narrator wished that he would fall silent.