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.
We will answer your questions in the following points:
1- The abrogation (of some verses of the Quran) is a confirmed matter, as there are some Quranic verses whose recitation was abrogated but their ruling is still effective, and some whose ruling was abrogated and their recitation remained, and some whose ruling and recitation were both abrogated. For details in this regard, please refer to Fatwa 84587.
2- In principle a Muslim is innocent and it is not permissible to accuse him of having committed a sin without evidence, especially if the accusation is about Zina while the person who is accused is known to be a good and righteous person, so this is more serious and the prohibition is even more confirmed. For more benefit, please refer to Fataawa 87225 and 85285. Hence, it is an obligation to admonish those who accuse people of Zina, and remind them of Allaah; and if they have no evidence about this accusation, then it is considered 'Qathf' (character defamation or slander) which is one of the grave major sins. If this accusation is not proven against this man, then the prayer behind him is valid; and even if it was confirmed that he had committed Zina, the prayer behind him does not become invalid, but it is more appropriate not to pray behind him. For more benefit, please refer to Fatwa 91299.
3- We are not aware of those symbols which you mentioned, so you may ask about them and their ruling some scholars in the country where they are used.
4- It is desirable to say "Inshaa’ Allaah" for things that one wants to do in the future, but this is not an obligation. So, it is permissible not to say it. But one should not refrain from saying it just because the person to whom he is speaking is not a Muslim, unless he fears that mentioning it would lead to a greater harm.
5- The Hadeeth is clear that not mentioning Allaah in a gathering will be a sorrow on its people, but as far as we know, no curse is mentioned in regard to those who do not mention Allaah in a gathering. For more benefit, please refer to Fatwa 90927. In principle, this Hadeeth is general and it applies to every gathering.
Allaah Knows best.