Standard Ranked puts you on the competitive ladder, using only the cards available in the Standard format at the moment, but if you have more cards from previous sets, choose Historic Ranked to have the freedom to use all the cards you want
The ability of giving all your other creatures +1/+1 is vital for Selesnya, an archetype thats trying to get as many creatures onto the battlefield as possible
However, there is no single answer to which game is the best
They can counter one, but the rest will still resolve often only marginally limiting the impact
To return to Cardhoarder, you would go to your account page on their website and request to return cards
Flashback More of an aid against floods where the high flashback costs are much more worthwhile things to be doing (than the nothing you might well otherwise be getting upto)