Setup real time mysql replication for portabilling master and slave servers

VoIP provider company is using Portaone product Portabilling as a billing system (version MR24).

Portabilling installation schema consists of 5 dedicated servers:
Master
Slave
Replication server
Configurator
Radius machine

Slave server hosted at OVH (dedicated server with 2 disks) had issue with one of the disks, which was replaced.
During replacement, the second disk was moved and it got out of the raid, which was not noticed by technician who did the replacement.
As a result, after booting the server from HD to get the new disk synched, server failed.
Mysql stopped (probably to avoid corruption of master mysql).
The mistake was found and corrected weeks later.

Technician tried to copy the content of disk2, which had slave info.
After copying, turned out that 2 partitions out of 3 are corrupted.
Corrupted partitions have been restored from tar archives.
Slave server has 2 versions of Portabilling (MR23 and MR24). Server is up now with 2 disks, which are synched.
Master and slave servers have real time replication. Replication is not done yet.

The project is to cooperate with the technician to complete the replication and to put slave up, making sure nothing will break or get corrupted on master DB.

Here is the list of tasks (the tasks with “? ” needs to be answered by the freelancer):
1. Check slave server and make sure it has everything (3 partitions) and when putting mysql replication up, it is not going to crash or corrupt master mysql database.
2.Currently https interface is showing error 500 (probably due to mysql replication)?
3. We have list of mysql users for slave (but not the privilege table).
Given the fact, that slave server can become the master one, in case of issues, the privileges for mysql users on both servers must be the same?
4.Except mysql users and Porta-billing database, is there anything which is missing to complete the mysql replication and put the slave server up?
Or DB is copied and mysql users are inserted/copied and that is enough?
5. Internal (employee) usernames and passwords to get to https interface are stored in partition configuration files?
6. Master Bin files needs to be copied from master to slave, or when replication setup is done it will synch with the last one and we do not need those on slave server?