If the code you're working on is changed by another player, it kicks you out of the code-breaking minigame. You can try again straight away, but the code will be different.
It's an interesting experiment I think. One player took loads of the central nodes for team red. I spent a couple of minutes cracking the first one, then tried that code on other nodes. It worked, he hadn't bothered to change it between nodes. There's a lesson about using the same password for everything in there somewhere!
It really hasn't had many players. I tend to leave myself logged on if my computer is on, but I haven't seen anyone join at least at the times I've been checking. You can see how many people are playing in the score panel - it shows the number of people in each team, and the total online.
You're right, the Mastermind/Wordle game is too hard, but you can use psychology too. In times where there aren't many players, you can try the same codes on all the same coloured nodes like I did. With more players, you can see the code someone is using while they're attacking a node, so if they move away you can remember it and take it back when they aren't looking. There's all sorts of subtlties, and there's almost certainly aspects I haven't considered too - this is the first realtime multiplayer game I've built!