You still have a replication agreement, and until its removed you will keep seeing these messages.  However it's not related to this issue though.

Good to know. I hope there is a way to force removal of that agreement.
- sometimes, but not always, this log also shows:
ERR - bdb_version_write - Could not open file "/dev/shm/slapd-HQ-SPINQUE-COM/DBVERSION" for writing Netscape Portable Runtime -5950 (File not found.)

This might happen after a system reboot.  It should be safe to ignore as long as the server still starts :)

Again, good to know, thanks

So looking at the error log it looks like the server is started.  Schema compat plugin is doing its initialization which is very resource intensive, but the server should still be working.

Try doing a ldapsearch just to see if it's responding:

ldapsearch -H ldap://localhost:389 -b "" -s base -D "cn=directory manager" -W

Ouch, I don't have the directory manager password with me at the moment, I'll have to wait till tomorrow when I go to the office.
The server is up and listening:

# netstat -tulnp | grep 389
tcp6       0      0 :::389                  :::*                    LISTEN      3575/ns-slapd       

However, it's not just a slow start. 
I can start all the other services via systemctl, so things seem ok, but when much later I do ipactl stop I get:

# ipactl stop
Failed to read data from Directory Service: Timeout exceeded
Shutting down


So, it's really not cooperating.