$ man opam-pin
-n prevents immediate reinstallation of the package (and all its reverse dependencies).
I do not understand why the pin commands failed in your case. This failure is the reason for the subsequent errors you encountered.
That won’t be necessary as the packages in question have now been updated on the opam repository and should be installable out of the box. Therefore it won’t be necessary anymore to pin them. You can unpin them by running
$ opam unpin <package-name>.
The perfect place to publish information on package compatibilities is the opam file of package releases. Once a pull request has been created on opam-repository, releases are generally published reasonable quickly, more often than within the day. In this case multiple packages that hadn’t been maintained for months had to be updated consequtively, which is why whole matter took a bit longer than usual.
I sincerely hope that the packages in question will not fall again into the state of disrepair that you had to witness. In any case, I will henceforth do my best to publish releases frequently and will ask the rest of the ocsigen team to do so as well.