1. It's not invading a thread when discussion is already turning towards that. A thread about Nino being the best character ever is about Nino being the best character ever. The discussion should be about whether she is or she is not. It is not invading the thread to say "Nino is not good for X reason". Likewise, if someone makes a comment in a thread about something, and someone replies to them, it's not invading any more than the original comment probably was.
2. Eh. I don't really know you, as I haven't been in the FE boards for a while. But really, it's only some people. Who knows, maybe they do have a reason.
3. In my experiences on those threads I've found the discussions are usually calm and friendly. Again, this is a bad argument, because IF IT WERE TRUE, you would essentially be saying "Some people who debate are assholes therefore tiers make people assholes" which is complete and utter lunacy. People are assholes in FFtF, people are assholes in General. People are assholes in lots of different forms of discussion.