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!).