While your frustration is 100% justifiable, not saying in any way that it isn't, how you handled the situation might be why you end up getting reported. No matter how politely you think you are being, things like "...and been useful..." will pretty much always sound passive aggressive and rude to the person you are addressing. It also doesn't help matters if you go on the offensive trying to find out who their friend is, and basically attack them for queuing up with someone you consider a troll.
Without seeing the logs of what was said, it's impossible to say whether you were being toxic or not. But I have seen countless people who end up in your same situation who let their irritation get the best of them, and do end up being toxic in response to someone who isn't playing the way they want them to. It starts off with "why didn't you play someone useful?" but progresses, until every few minutes there's something else like "can you actually do something besides stand at the turret and laugh?" "report this troll nunu" "f*^%$g trolls" etc. It becomes harassment. Again, I'm not saying this is what you did personally, as I don't have the logs.
As far as getting multiple reports in the match, that is one reason why Riot has the system set up the way it is regarding reports from a premade group. Because they are duo queued, their reports don't carry the same weight as if they were two random players. And they are capable of being reported for calling for reports in all chat. Riot considers that to be harassment, and as a player, you don't have to deal with it in your games. But the way to handle it is to just /mute them and report them. If you have a troll support, don't get drawn into arguments, or start snapping at them, because then you are breaking the rules yourself. Just report them. When they reach the reporting threshold, they will be reviewed for punishment. You may not see the alert, as you only get that if you are the report that triggers it, but that doesn't mean that they don't get punished for, in essence, ruining people's games.
