You may try to remove manually everything again, and to create the mysql user by yourself:

sudo bash
niutil -create / /groups/mysql
niutil -createprop / /groups/mysql gid 401
niutil -create / /users/mysql
niutil -createprop / /users/mysql gid 401
niutil -createprop / /users/mysql uid 401
exit

Then try to perform the installation again.

You may try to remove manually everything again, and to create the mysql user by yourself using this guide.

Then try to perform the installation again.