Okay, so it's a couple days later. I finally have some time to actually investigate and solve the issue.

My toolchain was out of date. That is, the one that worked fine. I assumed, since nix did not use rustup and genuinely gets its own rustc for rustPlatform that it might be latest stable. I was rocking 1.72.0 and it worked fine, and the latest is 1.74.0, of course it'll work! Nope. rustPlatform.rust.cargo.version is actually 1.70.0 and if i try to build niri like cargo +1.70.0 build it actually fails with the same error messages! I tried every release version after after this one by one, and installed like 4 new toolchains on my computer for that. There's probably a better way to do it. I narrowed the two errors down to these changes:

Cargo 1.71.0 patchnote: "Allowed named debuginfo options in Cargo.toml"

Stabilized APIs in 1.72.0: impl<T: Send> Sync for mpsc::Sender<T>