For all your questions simple answer would be ~yes
There is plenty if guides around rAthena just search and find which one is more suitable for you!
Here some of them.to start
Don't blame rAthena for your own mistakes ,
https://github.com/rathena/rathena/blob/master/sql-files/upgrades/upgrade_20170215.sql
You probably missing this table
rAthena is huge. There is new members joining everyday. That’s a lot of data for to store, so to lighten the load, ips application(the forum web application we use to run forums) compresses your images. This makes them smaller in size, and it is also depends on your image quality
Basically you want to disable duel clients?
If yes you can diff your client with disable multiple clients,
If you want to limit your multiple client login to two clients only you can do it by your game guard if you using one