Similarly, the high availability of source code may lead to malicious instances, actors, and/or back-end modifications that would favor specific instances resounding consequence throughout the Fediverse.
Historically availability of source code has prevented that sort of thing since forever. Plus you can’t favor a specific instance, that’s the beauty of the protocol. It’s like saying google can favourite a specific email provider, they can’t, if suddenly Gmail stops receiving or sending emails to random domains people would just switch boats because you can register on any of the other email providers that don’t do that. Gmail can collect your data and all, but all data on Lemmy is public, so there’s no need to mess with the source code to gather data.
So what are you worried about? Mods moderating content in ways you don’t like? That will happen on any platform that allows moderation, and you don’t want to use one that doesn’t (plus it has nothing to do with the open source nature of the server, and you can jump to another community with different mods). Maybe you’re worried that malicious software will run on your phone? That’s more likely to happen with a closed source software, if you’re truly paranoid about these things you would have a full open source phone with a custom OS without google components flashed into it, I can see that you’re not on that level since you still don’t understand that open source is needed for transparency. Or maybe you’re worried the server itself will host malicious content? Any server can do that, servers that host things people write will always be able to host malicious content, it’s not hard to link to an external website or provide malicious scripts or files, just don’t click on random links or download random things from strangers online and you should be mostly fine.