On the topic of calling for reports, while it isn't necessarily something spelled out in the code, it can create a negative environment, not only for the person being accused but for the rest of the players potentially depending on how feverous the one requesting is.
Now while that doesn't mean all "Report Player X's" are unjustified its generally considered better to not go panhandling for them, particularly from the other team if its something that isn't clearly visible to them. Yes they may have a valid reason, but that is in a case by case basis and there is nothing that says someone cannot quickly pick a champion in this case.
If I may lets put this into another sort of group game for a moment, how would you feel if you were going to play something akin to laser tag or paintball, and you had someone on your team you were randomly assigned with crying for everyone on your team and the enemy team to report you to the referee or some such. Ignoring what the reason may be, lets say they did this often, maybe every other game. What is the likely result. Chances are the team may start to argue about such, or at the very least the accused and the one crying out. This then spills out into shouting which the other team now has to put up with, Ignoring weather or not they did do something wrong for the moment, now the other players are having to deal with two players arguing over something potentially and may even join in themselves, the end result being a game that has become a bicker fest and with a potential for both sides to feel like they wasted their time, That is not a good thing and that is why its generally frowned on to be calling for reports without an good and obvious reason.
In short, while this isn't precisely a rule by any means, I do think its a good rule of thumb when judging a case to keep in mind "How would this effect me if I was playing" if it would have effected your ability to enjoy the game in a way outside of the game's design chances are its worth considering something to frown apon if done in excess, no one wants to have two players on their team starting an argument due to report bickering.
In this case that thankfully didn't happen, but considering There is no hard proof that the other players did anything that is breaking the code in those two cases.
1: Automatically picking Garen and Locking is not a crime, it can be bad for your team certainly but without the ability to see if he locked in before or after everyone else and weather or not its a normal game or ranked, its hard to say if the team didn't adapt to him or he didn't adapt to the team.
2: Nautilus has done nothing in this game as far as I can tell, at most, he either opted to or has gotten forced due to team comp into being an aggressive support lane for Ezreal (The Accused) and from what I can guess Ez was simply unhappy that Naut took a few creeps from the mid game and under performed and took it out on his team mate to want to report him in the end, what for, he doesn't specify ever just that he should be reported for some undisclosed reason.
It is kind of like saying you're going to call the police when nothing truly happened that is a crime, It creates a bad environment for those who are innocent and I don't think anyone would be too fond if they got accused for something like intentionally feeding when they just had a bad game and gave up a few kills because they got focused repeatedly.
TL: DR Calling for a report isn't a rule in the code, but needlessly calling it and possibly causing a negative environment for the other players is frowned apon and IS breaking a rule if it goes too far.