I'd strongly recommend revising your "mandatory" policy. Running such an old release is an exceptionally bad idea, replication aside.
For sure. It's kinda lazy rule, as we suppose the packages are released with Debian in a reasonable version.
Why would slaptest complain? I clearly stated it's using the default size, so clearly it has a configured value. This is explicitly documented in the slapd-mdb(5) man page.
Yes, you did, but for me maxsize 7516192768 would be enough. I'll dig deeper in the docs.