All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, saying that each country should act according to its sighting of the crescent, or that all countries should abide by the sighting of any other country, is a matter of difference of opinion which is a considerable one. However, this should not be a reason for division and dissension among Muslims, and magnifying this matter is in general due to the lack of religious knowledge. In any case, we will answer your questions in the following points:
1- Saying that Kurayb was not a trustworthy witness is not a correct statement; rather, he was a trustworthy person according to the consensus of the scholars. An-Nawawi said: “They (the scholars) agreed on his trustworthiness and Al-Bukhari and Muslim, reported many narrations from him.”
2- Saying that what Ibn ‘Abbaas said is just an Ijtihaad (i.e. exerting his personal judgment) and it was not attributed to the Prophet is refuted by his statement “The Prophet of Allaah ordered us…..” according to the understanding of those who say that each country has its own sighting of the moon. Al-‘Atheem Abaadi said: “…The scholars who said that the people of a given country are not obligated to follow the sighting of other countries stick to the Hadeeth of Kurayb, and their evidence is that Ibn ‘Abbaas did not act according to the sighting of the people of Ash-Shaam (Greater Syria) and at the end of the Hadeeth he said: “This is what the Prophet ordered us (to do)”; therefore, this is evidence that he learnt from the Prophet that it was not an obligation for one country to act according to the sighting of another country.”
3- Claiming that Kurayb and the people of Ash-Shaam were all wrong is a claim without evidence because in the same Hadeeth, Kurayb said: “…While I was in Ash-Shaam, we saw the crescent on the night of Friday (i.e. Thursday night) …and I said the people saw it and they fasted (according to seeing it) and Mu’aawiyah also fasted….” Therefore, they saw it with their own eyes.
4- As regards saying that Ibn ‘Abbaas did not seek two witnesses, then the answer to this is that Ibn ‘Abbaas was not in need of this, because even if two persons witnessed that the people of Ash-Shaam fasted on Friday, then Ibn ‘Abbaas would not agree with them because he was of the view that each country has its own sighting. The one who needs to bring two witnesses and should seek them is the one who wants to prove the information to be true. It might be that you just wanted to say that Ibn ‘Abbaas did not act according to the statement of Kurayb because it was a testimony of one witness only and not two witnesses and that he did not reject it because each country has its own sighting and some scholars are of this view. However, this (saying that Ibn ‘Abbaas did not act according to the statement of Kurayb because it was a testimony of one witness) is refuted by the apparent meaning of the statement that he (Ibn ‘Abbaas ) was of the view that each country has its own sighting. Al-Mubaarak Poori said: “According to this statement, Ibn ‘Abbaas did not act according to the news of Kurayb because it was a testimony and a testimony is not confirmed by the witness of one person, but it appears from his statement that he did not reject it because of this; rather, he rejected it because the ruling of the sighting does not apply to the people who are in far away lands.” This is also refuted by the fact that Ibn ‘Abbaas was one of those from whom it was narrated that the beginning of the month of Ramadan is confirmed by the testimony of one trustworthy witness. Indeed, it would take too long to mention what was narrated in this regard.
5- In regard to saying that the statement of Kurayb is just a news and not a testimony, then it appears that you misunderstood it, because what An-Nawawi mentioned in this context is that some people were of the view that the news of Kurayb was a testimony and a testimony is not confirmed by one person, however, he clarified that Ibn ‘Abbaas did not reject it because of this. An-Nawawi said: “Ibn ‘Abbaas did not act according to the news of Kurayb because it was a testimony and a testimony is not confirmed by one person, but it appears from the Hadeeth that he did not reject it due to this; rather, he rejected it because the ruling of the sighting does not apply to the people who are in far away lands.” For more benefit, please refer to Fataawa 115026, 86833 and 83817.
Allaah Knows best.