Bedrock is a cool concept, but it can become quite messy - installing just a couple of major packages from different distros could easily make a spaghetti of your system (superficially anyway). It's generally unlikely to actually break your system though, since each subsystem is sorta isolated into what they call a strata, something like a hacky poor-man's container/chroot. Whilst in my limited testing I haven't run into any breakage issues, the whole strata thing comes across as flaky and unsupported, sometimes leading to weird/unexplained behavior that you may need digging into.

If you like the idea of Bedrock though, IMO, a better and more practical/cleaner approach would be BlendOS, which uses proper containers to isolate packages from different distros, so there's no chance of breaking your system, and you could use standard container toolsets to identify/manage processes and packages from different disros. BlendOS is based on Arch so you have access to the latest packages by default, but it's also immutable, so the base OS is much more stable than Arch. So, you have most of the advantages of Bedrock, but with none of the confusion and mess.

I've never tried Bedrock myself, but was intrigued enough when I learned about it to read up. (So take my words with a huge grain of salt.) I believe that you have it right in your first point: you can run a kernel and drivers from, say, Debian stable while running cutting-edge rolling release userland applications from, say, Arch. And if you want a few slower-moving applications you can get those from, say, Ubuntu while still running the rest of your system as Debian and Arch. And if you need something really obscure that you can only find in a weird Gentoo overlay...

As to your second point, yeah, you can go with SystemD from wherever or OpenRC from Gentoo or Runit from Void or whatever else you want.

But really, I suggest you try out Bedrock in a VM and find out for yourself. If it works like you want it to, then go to town on your bare-metal install (after backing it up first, of course!).