Is leaving a negative review for a Muslim-owned restaurant considered backbiting?

Are Muslim community festivals and suhoor fests permissible in Islam?

Not categorically haram. Declaring a large Muslim public gathering haram simply because some may act improperly is an overreach. Whether attending is advisable depends on your level of piety and intention. For the highly religious, spending Ramadan in worship is better. For those at lower levels of practice, it may actually serve as an iman boost. Actions are judged by intentions.

Is it haram to talk to the opposite gender in Islam? What are the limits of male-female interaction?

Talking to the opposite gender is not haram in itself — no scholar has ever said so. What IS explicitly haram by hadith is khalwa (private seclusion alone with a non-mahram). The principle of sadd al-dhara'i is valid but culturally sensitive. Context, intention, and content all matter.

Is it permissible for Muslims to celebrate religious holidays of other faiths like Christmas or Diwali?

Muslims do not celebrate religious festivals of other faiths. However, Muslim converts may attend family gatherings on Christmas to maintain family ties, as long as they avoid religious rituals.

Is it permissible for Muslims to celebrate Halloween?

Halloween should be avoided due to lingering associations with demons and the supernatural, but it does not constitute shirk or kufr. Muslims who participate out of ignorance should be treated with empathy.