Unfortunately the usrmerge developers seemingly opted for going about this in the worst way possible, so the installation of usrmerge is likely to fail successively, requiring some amount of operator hands-on and reinstallation. Once installed, it gets replaced with the usr-is-merged package as indicator or flag for apt/dpkg to not install usrmerge again, ever.

All in all, it's better and almost necessary (esp for avoiding that operator hands-on) when shifting to excalibur, to install usrmerge on daedalus, i.e. before actually shifting. There is no technical difference difference between having or not having silly-links, except for the cases where a package installs binaries with a /bin path and then uses them on a /usr/bin path or vice versa (looking at you, busybox), in which case silly-links are required.

If you want to set up silly-links yourself you only have to make sure that the pathnames /lib* get copied and replaced with their links in "atomic steps"; i.e. you cannot use a shell script that separates "rm /lib" from "ln -sTf usr/lib /lib" specifically because the script intepreter's running of "ln" will require that the ELF-loader is found on its "/lib/.." path. (It's pretty obvious how to work around that though, for example by using busybox-static or , say, newlisp).

Running with Daedalus on my workstation, I have set up two VMs with Excalibur based on Daedalus cli installations about 6 months ago. My first step before changing the sources to Excalibur was to run usrmerge as it is recommended now in the documentation. And both of them work well.

Did you not see this link that I posted yesterday?

I've seen this if you're worried. I just figured that my main system continued to work because it looked like I did not have this package installed. However, I anyway have /bin, /lib (/lib32 and /lib64), and /sbin folders merged with /usr.

One possible reason might be the package 'usrmerge'.

That's it! I got dpkg errors related to usrmerge, ntfs-3g and sane-utils.

I installed Devuan Daedalus on the virtual machine and tried to upgrade it to Ceres, but it was bricked

One possible reason might be the package 'usrmerge'. These days it should be installed before changing to ceres.

The solution will come some day.

If you are ready for the unexpected, try Excalibur or Testing, and for the often broken, try Ceres or Unstable. Freshly updated/developed packages first land in Unstable, when they have sort of proven to generally work, they may be forwarded to Testing.

Testing some day becomes Stable, but currently this is about 2 years to go.

You only should use Testing if you are ready to accept uneven ground and if you are willing to contribute to this phase, or if you have a good reason to do so, i.e. you have so new hardware that its supported in Testing only. Something for the experienced user. And Unstable will be even rougher.

Just my two cents ...

Testing (Excalibur)  and unstable are very unreliable at this time partly due to the usrmerge. Have a look at the announcement:
UsrMerge Status: Testing/Excalibur and more

Other issues are also discussed on this very forum.