Learn Turkish: Pikniğe gidemedik, en azından evde iskambil oynarız. - We couldn't go to the picnic, at least we will play cards at home.

Yes, all three are perfectly interchangeable here. They all act as concessive markers that present a minimum acceptable alternative. Because the ideal plan of going to the picnic fell through, playing cards at home is offered as an acceptable backup. You could easily say bari evde iskambil oynarız or hiç olmazsa evde iskambil oynarız without changing the meaning.

Why does the translation say we will play when oynarız uses the aorist (simple present) tense instead of the future tense oynayacağız?

In Turkish, the aorist tense (using -r, -ar, or -er) is heavily used for spontaneous decisions, offers, or casual suggestions about the immediate future. It feels much like saying 'we can just play' or 'we'll play' in English. Using the strict future tense oynayacağız would sound like a firmly scheduled, definitive plan, whereas oynarız perfectly captures the casual, on-the-spot nature of a backup idea.

Why is it just iskambil and not iskambiller if the English translation is plural (cards)?

In Turkish, the names of games don't take the plural suffix, even if they involve multiple items. You just say iskambil oynamak to mean playing the game of cards. If you were to say iskambiller, it would sound like you are referring to the physical pieces of cardboard themselves rather than the activity.

I still mix up the negative potential sometimes. How exactly does mean we couldn't go, rather than we didn't go?