I have a question regarding la3n.

Firstly is la3n halal?
Secondly when am i allowed to send la3n on someone?
Thirdly, if someone claims to be shia but openly says things such as hadeeth al kisa is bid3a, the walk of arbaeen is an innovation etc and this persons idea is causing disunity in a community am I allowed to send lanat on then person?

La’an is a dua which is requesting Allah to withdraw His mercy from someone. It is mentioned many times in the Quran. It should be sent on enemies of Allah and the Ahlulbayt
You should not say it against someone you disagree with only even if they’re wrong. The only way they’re to be included in la’na is if they’re clear enemies of God and Islam.