None of the others are as deeply integrated into everything as systemd, they pretty much just handle starting things up so dropping in a replacement should be fairly straightforward. At least, it was until everything switched to systemd. Which is probably my biggest issue with it: that it integrates to the point you can't replace it anymore.

Honestly I don't know. I just know that desktop environments and a lot of other packages have hard dependencies on Systemd, at least on Arch and Debian based systems. Those packages include: base, flatpak, polkit, xdg-desktop-portals, and vulkan-intel. So yeah, it's nearly impossible to not break anything.