Brother Hani, why do you want me guessing??? We're not playing a game, are we??? Anyways, lets take this discussion forward. Kufr has to do with Nabuwath and Shirk has to do with Tauheed. If you reject Nabuwath, any part of Nabuwath or the status of any Messenger then, this would result in Kufr.If you worship anyone, anything apart from Allah or associate anything or anyone to Allah then, this would result in Shirk.
Now not to believe in the status of any companion or having doubts about their faith and belief, or categorising their status and position or cursing any companion or even humiliating, insulting, slandering, swearing or what ever else you would like to add in, how and in what way is this Takfeer???
Salam,
Okay, since you don't want to contribute your thoughts, I'll give you two examples InshaAllah at the end of this post.
Now before we begin, I'd like to clarify that I (as well as you) are not qualified to discuss topics as we've never researched them or read related books.
Regarding Kufr, there are popular books which speak on the boundaries and laws of Kufr, who is a Kafir and who is qualified to make a declaration of Takfeer.
Now you stated that Shirk has to do with Tawheed, this is true. Then you said that Kufr has to do with Nubuwwah, I say everything has to do with Nubuwwah even Tawheed, because if you do not believe in Muhammad (saw) then you don't believe in the Qur'an and the Tawheed.
Regarding Kufr:
You said that insulting the companions or making Takfeer on them does not constitute Kufr. I say it can indeed constitute Kufr.
How?
Well if you believe for example that Sahih al-Bukhari is a book containing sayings correctly attributed to the Prophet (saw), and in Sahih Bukhari the Prophet (saw) says: "Abu Bakr is from the dwellers of heaven."
Then you accuse Abu Bakr that he is an evil apostate because of his ruling on Fadak, that means you rejected the Prophet's (saw) words and promise making you a Kafir.
Similarly, if you believe that whatever Ja`far says in al-Kafi is attributed to the Prophet (saw), and Ja`far says: "`Ammar is a believer who will enter heaven." Then you disagree saying, "No he's in hell because he obeyed `Umar and worked for him."
This makes you a Kafir because you rejected what is correctly attributed to the Prophet (saw) in al-Kafi.
Now you ask, well you're a mainstream Muslim and according to your texts Abu Bakr is in heaven, but me as a Shi`ee I don't believe in your books and in my books he's an evil apostate, how does that make your view of me? Am I a Kafir?
I personally believe in this case, that you are committing acts of Kufr but are not a Kafir. Meaning, you are sinful and will be punished but you're not a Kafir as you don't believe in my books in the first place. Thus, a deviant Muslim is what you become, you have the rights that any Muslim has.
Is Kufr limited to Nubuwwah? The major Kufr is known but there is minor Kufr, or as we said an act of Kufr that does not render its doer as a non-Muslim.
For instance, the Prophet (saw) said: "Whoever insults one's lineage or wails on the dead has committed Kufr."
He (saw) also said: "Whoever seeks a fortune teller has done (an act of) Kufr."
Similarly, we find that there is Shirk Akbar and Shirk Asghar, a man who believes that `Ali is God on earth has done Shirk Akbar such as the Sabaa'iyyah and the Nusayriyyah and other Shia sects. He becomes a disbeliever by doing so.
As for committing an act of shirk, it can be like this Hadith where he (saw) says: "Whoever gives an oath/promise by other than Allah, he has made Shirk."
This can include Shia in our days who say: "By `Ali, I did not do this" or "By the Wilayah of Sahib-ul-Zaman I love you."
This is an act of Shirk, doesn't make the doer a polytheist or non-Muslim, but is still sinful and he will be punished if Allah wills.
Finally, you want to see examples of why some Muslims may attribute Kufr/Shirk to Imamiyyah, I will give these two examples, starting with an example of Kufr:
عن الحسن بن الحسين بن بابويه ، عن عمه محمد بن الحسن ، عن أبيه الحسن بن الحسين ، عن عمه أبي جعفر بن بابويه ، عن القطان ، عن ابن زكريا عن ابن حبيب ، عن ابن بهلول ، عن أبيه ، عن أبي الحسن العبدي ، عن سليمان ابن مهران ، عن عباية بن ربعي قال : قلت لعبد الله بن العباس : لم كنى رسول الله صلىاللهعليهوآله عليا عليهالسلام أبا تراب؟ قال : لانه صاحب الارض ، وحجة الله على أهلها بعده ، وبه بقاؤها ، وإليه سكونها ، ولقد سمعت رسول الله صلىاللهعليهوآله يقول : إنه إذا كان يوم القيامة ورأى الكافر ما أعد الله تعالى لشيعة علي من الثواب والزلفى والكرامة ، قال : « يا ليتني كنت ترابا » أي يا ليتني كنت من شيعة علي وذلك قول الله عزوجل : « ويقول الكافر يا ليتني كنت ترابا »
[From al-Hasan bin al-Husayn bin Babawayh, from his uncle Muhammad bin al-Hasan, from his father al-Hasan bin al-Husayn, from his uncle abu Ja`far bin Babawayh, from al-Qattan, from ibn Zakariya, from ibn Habib, from ibn Bahlul, from his father, from abu al-Hasan al-`Abdi, from Sulayman bin Mihran, from `Abayah bin Rib`i, I said to ibn `Abbas: Why did the Prophet (saw) call `Ali as abu Turab(father of sand)? He said: "Because he is the owner of the earth, and the proof of God upon its inhabitants, and through him it remains (in existence), and because of him it remains stable (doesn't fall apart) etc...]
Source: Bihar al-Anwar 35/51.
And this isn't stuff Shia don't believe in, Sistani's official research center presented this narration as an answer to a questioner who asked "Why was `Ali called abu Turab?":
http://www.aqaed.com/faq/4720/When we know that the earth belongs to Allah and he sustains everything and preserves everything with his power, not `Ali:
{Allah - there is no deity except Him, the Ever-Living, the Sustainer of [all] existence. Neither drowsiness overtakes Him nor sleep. To Him belongs whatever is in the heavens and whatever is on the earth.} [2:255]
{Do you not see that Allah has subjected to you whatever is on the earth and the ships which run through the sea by His command? And He restrains the sky from falling upon the earth, unless by His permission. Indeed Allah, to the people, is Kind and Merciful.} [22:65]
Then an example of Shirk:
عن أحمد بن أبي جعفر البيهقي ، عن علي بن جعفر المدني ، عن علي بن محمد بن مهرويه القزويني ، عن داود بن سليمان ، عن الرضا ، عن آبائه عليهمالسلام قال : قال رسول الله صلىاللهعليهوآله : إذا كان يوم القيامة ولينا حساب شيعتنا فمن كانت مظلمته فيما بينه وبين الله عزوجل حكمنا فيها فأجابنا ، ومن كانت مظلمته فيما بينه وبين الناس استوهبناها فوهبت لنا ، ومن كانت مظلمته فيما بينه وبيننا كنا أحق من عفا وصفح
[From Ahmad bin abi Ja`far al-Bayhaqi, from `Ali bin Ja`far al-Madani, from `Ali bin Muhammad bin Mahrawayh al-Qazwini, from Dawud bin Sulayman, from al-Rida (as), from his fathers (as), the Prophet (saw) said: "On the day of judgement we will be in charge of judging our Shia, whichever of them has an issue with Allah then we will judge it and He will accept (our judgement), whichever of them has an issue with the people then we will take care of it for him, and whichever of them has an issue with us then we are the most worthy of offering forgiveness."]
Source: `Uyoun Akhbar al-Rida 2/57.
Now we all know that on the day of judgement Allah is the judge, and associating the Imams with Allah in judgement on the day of judgement is Shirk.
Allah says:
{But Allah will judge between them on the Day of Resurrection concerning that over which they used to differ.} [2:113]
{And indeed, your Lord will judge between them on the Day of Resurrection concerning that over which they used to differ.} [16:124]
So the above is just examples of typical Shia narrations that conflict with Qur'an and Sunnah according to Ahlul-Sunnah, thus the accusations against Shia of Kufr/Shirk.