Welcome to the forum DeenSlavi!
You are absolutely right that EA Studio is an awesome product. It will improve your trading by 100%.
1. The Portfolio eAs are only for MT4. Why? Because MT5 does not allow hedging with most of the regulated brokers.
2. In Full data optimization, you are using the complete Historical data you have. With the Walk forward you have the option to use OOS. This stand for Out of Sample. If you select 20%, that would mean that you are using 80% to optimize and the rest 20% will be simulated trading. This way you can see how the strategy performed as you traded it.