If someone was really toxic before the game happened, is my report still valid?

Pre- and post game chat aren't included in reform cards, but they are looked at when handing out punishments.

But it never hurts to send a ticket to support with the screenshots.

Yes. Add in the description what they said in pre-game lobby.

I need to know the answer too , I had once a Riven who went toxic against me for having an anime character name , that Riven flamed a lot , I couldn't mute her in champion select , then she was normal in during the match , didn't flame at all as if she knows that flaming in champion select isn't reportable

Riot has everything. Tell them what happened, and they will the evildoer.

It depends. Riot loves protecting game destroyers and punishing people who actually play to win. So if you're a really bad player and destroy every game you play, chances are Riot listens to all your reports and have already banned most of the allies you've played with.